Baccano! wraps up everything which concerns episodes 2 to 16, but the very first episode is a mash up of future events in the show, where there's a particular short scene where a blond girl slices Issac's ear off with a spear. The girl's name, Adele, is not even mentioned through the show as she would later appear in a future arc from the source material right after where the anime ends.
Full Metal Panic! resolves its main plot, but leaves hanging a number of plot threads, most importantly the precise origin of The Whispered (only resolved in the novels) and the Black Technology that seems to be encoded in their genetic structure. One might presume the new season of Full Metal Panic!, "The Second Raid", would answer some of the unresolved issues. It doesn't, and the source material is still going; while there's no absolute guarantee it'll be dealt with, resolving these issues would require making something up just to end the anime.
The original TV ending for Neon Genesis Evangelion didn't resolve much, and in fact the English edition has a hilarious Easter Egg where Spike Spencer, the English voice actor for Shinji, complained (in-character) about the sheer number of dangling plot threads that were never addressed. Many of these ended up being answered in The Movie finale, End of Evangelion.
Also see Variable Geo, and how it doesn't resolve the whole deal with the tournament it's centered upon.
Genshiken - both the anime and the manga, but for different reasons. In the anime, material was added for the second series to ensure enough was left for a third series, but there's no guarantee there ever will be one to finish the job. The manga has the "life goes on"-No Ending that is so typical for Slice of Life series, so it was almost inevitable that not everything got wrapped up.
Parodied / Lampshaded (like a bazillion other tropes) in Ninin Ga Shinobuden. Onsokumaru and another character have an ominous conversation about Shinobu's hidden destiny. In the last episode the characters realize they forgot all about this plot thread and hastily construct a Magical Girl story to cover.
Mari's fate in the Blue Drop anime after her loved one Hagino dies is never addressed, not even in the huge time skip at the end of the series. This is especially egregious, since Hagino sacrificed herself to save Mari's life, so it would at least have been nice to know how Mari dealt with it.
A variety occurs in Koi Koi 7, where everything is set up for the final battle with the Big Bad—only to have everybody inexplicably relax on a beach in the last episode. An attempt is made to explain what happened in a short DVD-only episode, but it's still one heck of a sudden turn.
Spiral is a mystery series, and its main mysteries are "What are the Blade Children?" and "What happened to Kiyotaka, Ayumu's older brother?" The anime never answers either of these questions, as the manga hadn't yet by the time it finished production. An offhand comment by Kanone might or might not imply that Kiyotaka is dead in the anime continuity, and another compares the Blade Children to cuckoos, but that's it.
Most of Piano is about Miu composing a piece for her piano recital. At the big moment, the story simply cuts off when Miu starts playing, without stating how the recital went or what happens between Miu and her love interest afterward. Sure, the main point is Miu's decision about her music and her life, but it would have been nice to get some more closure.
A particularly infamous example of this is Ranma ˝; the anime version, already criticized for being stuffed to the brim with Filler that had absolutely no bearing on the original manga plot, concluded a full three years before the original manga did, cutting out roughly a full third of the overall plot. For those without easy access to the manga (which, as a note, did not finish officially releasing in English-speaking countries until 2006, ten years after the conclusion of the manga in the original Japanese), this was kind of a huge kick in the pants.
Code Geass actually resolved its main premise, but several subplot threads were apparently lost in the rush or deemed less important somewhere between seasons as the staff had to alter their original plans due to an unexpected time slot change. They include the true nature of Suzaku's superhuman abilities, C.C.'s real name and, perhaps most frustratingly for several viewers, any sort of official explanation for the Geass.
Martian Successor Nadesico does this twice at least in America- first, the series ends with only a bare acknowledgment of how the war ended, then they made The Movie which takes place after a Time Skip, doesn't explain the war's end either, and ends on its own cliffhanger (it was meant to be the first part of a movie trilogy which was never finished). The first hanger was only resolved through information which is All There In The Japan-Only Sega Games. The last episode of the series even has the audacity to introduce new subplots just to intentionally leave them hanging.
The first anime fell into this hard, mainly because it was supposed to go for two seasons, but the general lack of quality and pure Off Model-ness got it killed after one season, forcing a Gecko Ending and dropping a bunch of newly introduced plot threads. Not that two seasons would have come close to covering everything in the manga, but starting a bunch of B-plots and then ending the series two episodes later is pretty bad.
The manga hits the same issue (though much later on and for different reasons), trying to at least resolve as much as it can in the very last chapter and only succeeding partially. If you want to know who the Gravekeeper is, what happened to Arika or any number of other things like that, you won't get to know. And more importantly, who Negi ended with. In fact, we only got most likely candidates DIDN'T win, which is even worse than not getting anything at all.
Kare Kano: Everything was left hanging. EVERYTHING. The play that they built up for 6 or 7 episodes never happened, there was no culture fest, and the only way to actually understand what happened between Tonami and Sakura in the final episode was to read the manga.
At the end of Heat Guy J, Daisuke has been permitted to leave Judoh and go off on adventures, and Kyoko tells him I Will Wait for You (but only for three years), and he promises to come back. Off he goes, Riding into the Sunset and...that's it. Where did he go? What did he do? What became of Kyoko? And Clair? And the others? The world will never know. It appears to have been a Sequel Hook, but that sequel never materialized.
The anime of Berserk covers the Golden Age arc... and then it ends during the Eclipse. Since the series begins with a scene set after the Golden Age arc, it means the series never got out of the flashback.
Soul Eater managed to end without Eibon, the enigmatic (presumably) Bigger Bad behind Noah and the Book of Eibon having made an appearance outside of a flashback. Hell, the entirety of Noah, who has the Book of Eibon and Brew and 5 more copies of himself is Left Hanging. Only Noah-Greed was given a major role as a villain.
The World God Only Knows has the entire plot suddenly resolved with the abrupt revelation that a main character (Elsie) is actually a powerful monster and the enemy's ultimate weapon and her using her powers to produce some sort of Cosmic Retcon that makes the good guys win somehow. A very significant number of questions are left unanswered and plot threads unresolved; in particular Lune's whereabouts, what happened to Shoutarou Shiratori, and more importantly how Satyr was able to use Elsie for their plans and what will happen with the girls who still have the goddesses inside their bodies.
Issue #20, "Mekkano": the eponymous machine, which can break down any other machine for parts and add it to itself, is picked up in Earth Orbit by an alien spaceship, which takes it back to their base. Never seen or mentioned again.
A storyline in Power Pack heavily implied that Toro, the sidekick of the Golden Age Human Torch, was not only still alive, but was also the father of Frankie Raye. This never went anywhere, and the later Avengers/Invaders crossover confirmed that Toro was still dead.
Next Men ended with a cliffhanger in #30. Byrne had intended to conclude the story in a second series, but the collapse of the American comic book industry in the mid-1990s made it financially unfeasible for him to do so.
When Crossgen Comics folded, they promptly ended every single comic mid-story, right when the overall plot was reaching its apex. However, Marvel Comics announced that they plan on publishing Crossgen titles, although whether this means backlog or new content is up for grabs.
DC Comics' Star Raiders graphic novel was truncated to half its length due to a cancelled contract; many secondary characters and plot points were Left Hanging as a result.
The final issue of Captain America and The Falcon Vol. 2 ended with the implication that the Falcon might have been killed by Anti-Cap, as well as the real Captain America finding his abandoned costume fluttering in the wind. Word of God is that the Cliffhanger was meant to be a Sequel Hook for a solo Falcon series by Christopher Priest, but it fell apart when Priest left the project. When the Falcon next appeared in Ed Brubaker's Captain America run, he was back to wearing his costume, and there was no mention of his disappearance or the prior incident with Anti-Cap.
In one of the issues of the Daken solo series, the title character reads a news article about the new Captain America being outed as Bucky Barnes, the man who killed Daken's mother. He's shown looking at Bucky's picture with an intense stare, and there is a strong implication that Daken is planning on confronting Barnes. The book was cancelled before this fight could occur.
Runaways ended with Chase Stein getting hit by a car after chasing a girl who looked suspiciously like his dead girlfriend while the rest of the team contemplated making a deal with Chase's shady uncle. There was also some mystery about the sudden disappearance of Old Lace. When the team later reappeared in other books, Chase's near death was handwaved, and Old Lace's disappearance was explained in a way that raised more questions, but Gert's sudden reappearance was never explained, and Chase's uncle was never heard from again. To this day, there's never been an explanation for the sudden attack that kicked off the story's events, either.
The newspaper strip Little Orphan Annie ended in 2010 on a cliffhanger, as Annie was kidnapped by a war criminal called the Butcher of the Balkans, who intended to raise her like his daughter, while Daddy Warbucks was left to believe she had been killed. It was recently announced that this cliffhanger will be resolved starting in June of 2014, in the comic strip Dick Tracy.
Nero: This newspaper comic was published daily for over 55 years. Author Marc Sleen had to work so fast that he made zillions of continuity errors and plot holes that remained unsolved.
Link's New Look was meant to be as twice as long, but only the first half was uploaded due to backlash. Later averted as the author did finish the second half.
Films — Animated
In-universe example: In Toy Story 2, Woody learns his TV show from the 1950s, Woody's Round-Up, was left hanging during a cliffhanger in the final episode because low ratings caused its cancellation.
The animated The Lord of the Rings notoriously ended halfway through with no warning. There was supposed to a second movie to wrap things up, but it never got off the ground. The Return of the King was produced a few years later by Rankin and Bass, the company who made the animated version of The Hobbit; it did not go as well.
Films — Live-Action
Sky Captain and the World of Tomorrow: After Polly the reporter takes a photo of Joe, aka Sky Captain, he says that she left the lens cap on her camera, and her looks of joy turned into a big, big sad stare, and cut to black. Whatever happened to the falling animals, the city and their worries about the evil doctor that sent his robots, the people they rescued, and other stuff, is left to wonder. All part of the 30's serial feel of course.
Fate of the Phantasm series as the fourth film ends in a cliffhanger where Mike seemingly dies and Reggie enters Tall Man's world. That was in 1998 and fans are still waiting for a conclusion. And Angus Scrimm isn't getting any younger.
Another in-universe example: In Galaxy Quest, the last episode of the titular show when it was cancelled ended on a cliffhanger, with Captain Taggert shouting "Activate the Omega-13!".
Ferris Bueller's Day Off: Looks like we'll never know how Cameron's "little chat" with his father went, despite all the post-credits scenes.
W ends this way, literally. At the end of the film, after he embarasses himself by being unable to answer a reporter when she asks him what his biggest mistake was while in office, Bush has another baseball fantasy where he runs to catch a long fly ball which never comes down.
Scanners ends just after Vale and Revok merge into a single being (though not in a Body Horror sense), but there's still some ambiguity about which consciousness is more in control. Grand Theft Me of Revok by Vale would be the nicer possibility.
The Charles Sheffield sci-fi novel Cold as Ice follows this trope to ridiculous extremes as the writer tells the story of several, unconnected main characters. One of these, a standard End of the World plot, is resolved in the main story. The others are swiftly, and without explanation, dealt with in the Epilogue.
The final book of K.A. Applegate's Animorphs ended this way by introducing a new antagonist with absolutely no desire from the author to go further with it (she even admits to leaving the reader hanging in the afterword).
The reader never finds out who burned down Joe Bob Fenestre's house. For that matter, we never find out what happened to Joe Bob Fenestre at all.
In #41, the whole Bad Future thing turns out to be a mind exercise brought on by some other entity that is studying humans. It's not Ellimist or Crayak... So who was it?
Another series by Applegate that ended with the readers left hanging was Everworld, which abruptly cuts off with numerous subplots, and even the main story itself, mostly unresolved. Plot threads left dangling include: Brigid, the coming battles against Ka Anor, the fate of the Sennites, how the introduction of technology will affect Everworld, who will be the next mayor of Atlantis, the Great Scroll of the Gods, etc.
Harry Potter and the Deathly Hallows leaves the fate of the evil witch Dolores Umbridge up in the air after she is placed in a high level of the government after the Death Eaters take over. J.K. Rowling soon came out and said that Umbridge was prosecuted and imprisoned.
Raymond E. Feist's Riftwar Legacy series, which was left unfinished due to copyright issues, since two of the three books were novelisations on Riftwar-based video games. The events that presumably happen during the finale of that series have been referenced in later books, but it is uncertain whether the books will actually ever be written.
The Brothers Karamazov, on any number of plot points. Dmitri is still wrongfully in jail, convicted of a crime he didn't commit. Ivan is still suffering from insanity. The fate of Ilyusha's family is uncertain. Alyosha's fate is uncertain. We've all learned a lesson, though, so they don't need to be cleared up for us.
The books never officially explain what the Sugar Bowl secret is, they never explain who actually burned down the Baudelaire mansion... In fact, the series ends with a note that basically says, Real Life is full of mysteries. Get over it.
Also, the fates of the Quagmires, Captain Widdershins and his stepchildren, and Hector.
In The Neverending Story many subplots are deliberately left unresolved: "But that is another story, to be told another time."
Piers Anthony had a book called Mute which created an entire well-defined and intriguing universe, with complex characters and hinted-at half-revealed plans, ended it on an unresolved plot Cliff Hanger... and then dropped it. Word of God is that he's not going to pick it up again, ever.
In The Year of the Book and Honeysuckle House by Andrea Chang (novels for middle-grade readers featuring Chinese-American characters), it can reasonably be said that what is more or less considered the main plot is wrapped up, but a number of important plot themes are simply left hanging, such as, in Honeysuckle House, whether or not the father of the main character gets his Green Card to work in the United States. The Year of the Book at least has a sequel (May 2013), but no such luck for Honeysuckle House.
A Wolf In The Soul ends with Greg defeating the werewolf and returning to normal, but many other subplots, such as his parents' marriage and the reason he was infected in the first place remain unresolved.
Bonfires and Broomsticks, the second book in Bedknob and Broomstick, ends with Miss Price having gone off to the past and the children walking to the ruins of the house that she would have lived in, only to hear her disembodied voice tell them to get off of the cabbages.
24 has an unfortunate tendency to simply abandon important secondary characters and leave their fates hanging; examples include Rick from season 1; Miguel, and Lynne Kresge from season 2; Andrew Paige and (quite egregiously) Behrooz Araz in season 4; and former President Charles Logan in season 6. The hanging plot thread regarding Charles Logan was ultimately subverted it the long run, as he turns out to have survived his wound and went on to serve as the Big Bad of the final season.
Now and Again - final episode of the first (and only) season ends with most of the plot strands resolved and a brand new bunch just springing out in the last five minutes.
Star Trek: The Next Generation, in the "Groundhog Day" Loop episode. When the loop breaks, they find out that their collision partner is a ship — captained by Kelsey Grammer — that is a full century out of date, the USS Bozeman. It was never mentioned on the show again, but dialogue references to a ship named "Bozeman" popped up in both Star Trek: Generations and Star Trek: First Contact, and it was the subject of a (non-canon) novel. TNG also left the fate of Enterprise-C and the alternate timeline Tasha Yar hanging in "Yesterday's Enterprise". Did they succeed in their mission, or die senseless deaths? All we knew is that their passing back into the phenomenon, they restored Enterprise-D to it's original timeline, with only Guinan aware of the entire affair. It wasn't until much later (several seasons) that the fate of Tasha Yar was learned.
The 2000s Battlestar Galactica has a reputation for not leaving plot threads unresolved, though due to the sheer number of threads ongoing in any episode some tend to be dropped due to lack of time or neglect. Examples include Boxey in Season 1, who was left on the cutting room floor after two episodes, and "Shelly Godfrey", a Number Six hiding within the civilian fleet who is never seen again after her sole appearance. (Notably, Helo's storyline was intended to be abandoned after the pilot miniseries, but was maintained due to popular demand.) Apparently, Shelly Godfrey will eventually be explained, in "The Plan". However, the Opera House itself (not the visions), the original temple on the Algae Planet, and the exact nature of the Lords of Kobol seem destined to remain in the file marked Left Hanging. Not to mention "God". Many fans thought that Shelly Godfrey was just Head Six who had materialized herself to help Baltar, but The Plan shows that she was a real Six trying to discredit him. Cavil and another, cooler Six intimate that she made it too easy to discover her fake evidence because of Baltar's "dreamy hair".
Seems like we'll never find out now what was going on with that damn spy pen in Veronica Mars...
Firefly's abrupt cancellation left a number of plot threads dangling. Though many of them, particularly River's psychosis and the origins of the Reavers, were covered in The Movie, many more, such as Book's past, were left in the air. They're only resolved in the comics, Book in particular gets a trilogy devoted to his past.
Stargate SG-1 ends with an episode that makes no attempt to resolve any of its plot lines. Of course, given the many times they were Absolutely Finally About to Be Canceled and got renewed again, it could be that The Powers That Be didn't know it was for real this time. At any rate, a Wrap It Up movie, Stargate: The Ark of Truth, came out and resolved the Ori plotline, and was followed up by Stargate Continuum. A third movie was announced, but seems to be stuck in Development Hell.
And a canonical MMORPG, and concepts for another series... Interestingly however, Stargate: The Ark of Truth seemed to be quite deliberate in avoiding stating "Yes, the Asgard are dead and gone we're not going to retcon that." Events from the final SG1 episode remain, for instance the presence of the Asgard computer core, but there is no reference to the fate of the Asgard. Oddly though, they are stated as an intended playable race in the MMORPG, which takes place some time after the upcoming movies — supposedly a significant amount of time, despite, again, being allegedly canonical.
Stargate is a prime example of this trope in general. Especially in the first seasons there were a lot of plotlines opened that were just left hanging. For example Daniel's grandfather as ambassador with an alien super race is never again heard from. Or the time they found a device with the recorded knowledge of all the ancient races on a planet where the gate plunges into the ocean is never visited again as soon as the SGC has ships. You should think something like this would be of value enough to fly there, especially since the planet was very close to Earth.
Keeping with the Stargate tradition is Stargate Universe, At the end of the final episode Eli has decided to stay awake onboard the Destiny while everyone else goes into stasis, in hopes of fixing the broken stasis pod and joining them so the Destiny can begin is really long voyage to another galaxy, one where they are not constantly pursued by evil aliens and robot drones. It end with Eli staring contemplatively into space and no follow-up seasons exist to resolve matters.
Red Dwarf ends with a sci-fi cliffhanger involving Rimmer kicking Death in the groin with the rest of his cast members are in a mirror universe, while the ship was being eaten slowly by a genetically engineered virus.
The new 3-parter on Dave 10 years later does noting to resolve this, as it's set, well, 10 years later.
Lampshaded/subverted with the end of Season 2. Season 2 ended with Dave pregnant with twins fathered by a female version of himself from a mirror universe. Season 3 opened with a Star Wars Crawl that explained it all away, but was too fast to read.
The ending of the TV miniseries The Lost Room appears to totally forget about the fates of Detective Bridgewater and Dr. Ruber.
The Fox sci-fi series Space: Above and Beyond ended its one season run with an awful lot of loose ends left unwrapped. The show ended with one character presumed dead, two more falling in an escape pod into enemy territory, one reunited with his prisoner-of-war lover, and everyone else generally in limbo.
90210 started off as a series that involved both the old gang (the ones from the 90s) and a new fresh almost entirely unrelated (except for the half-sister of the franchise protagonist). For instance, it was revealed that Dylan and Kelly had a son but then broke up, Brenda was sterile but ended up adopting, Kelly's mother relapsed in alcoholism and got cancer for which she'd die in the second season, but as the show changed executives, it was decided that it wouldn't rely on the old gang any longer (except for Kelly, who's now been downgraded to recurring supporting character), leaving the whole thing about what happened to Donna and David, their divorce, their kid, David's flowers and message in the air.
The final (British) series of Primeval ended with a battered, bloodied future version of Matt appearing him, warning him that there's still something he needs to fix, and that he needs to 'go back'. With the New World series mostly unrelated to the old characters, the movie languishing in Development Hell and the sixth series even worse, it is doubtful this huge plot thread will ever be picked up.
The popular sitcom Titus was canceled before they could write a proper episode to conclude the series. While the last episode was good and funny, it ended with Titus being put in a mental hospital for a few months as a stipulation for everyone to avoid jail time.
An episode of The Office (US) dealt with Dwight finding a joint in the parking lot and becoming more paranoid than ever as he tries to find the culprit. By the end this morphs into Dwight covering for Michael, who had accidentally inhaled some pot smoke the night before, leaving the question of how the joint itself got there unanswered.
It's revealed in a deleted scene that it's two employees from Vance Refrigeration.
Dwight never finds the man who flashed Phyllis, either.
Kamen Rider Kabuto: Who is Dark Tendou? Why was ZECT cooperating with worms from time to time? Where did the Hopper Riders come from?
In Phil of the Future, after Phil and Keely get together, the Diffys head back to the future, only to turn around for the cavemen Curtis. Then it ends forever.
The cliffhanger at the end of season 2 of LOST did not become relevant until near the end of Season 3. It was not directly referenced and resolved till the end of the fourth season. On the plus side, you can take it as evidence disproving that the writers are making it up as they go along.
Name a high-concept Sitcom from The Sixties. Lost in Space, Gilligan's Island, and many others were summarily canceled when the time came, even after a successful run and regardless of whether or not the show featured An Arc, simply because that's how business was done at the time. A lot of them eventually got Wrap It Up movies once they'd had enough time in syndication to get interest back up.
Perhaps David Lynch thought that by leaving every single subplot sadistically hanging on multiple cliffs that the fans would scream and cry for another season in which to see them all resolved. If such was the case, then the tactic didn't work as the show was canceled with little fanfare and with much grumping by the small devoted fanbase the show had. And apart from the subplots, the main plot wasn't really resolved with the "How's Annie!" ending either.
In the Cracker episode "One Day A Lemming Will Fly", Fitz actually spends the entire episode pursuing the wrong man for the murder of a child. The episode's entire resolution hinges on the fact that the child's killer will never be known - at least, until he strikes again...
Kyle XY left many plots dangling (although Word of God cleared up a few of them). In fact, the finale actually introduceda new plotline in the last few seconds of the episode!
The Sarah Connor Chronicles ends with John being sent into the future with a T-1001 where Kyle is the leader of the resistance and nobody has heard of John Conner. Also Sarah remains in the past, Cameron's human double is alive and John Henry the computer program, which is apparently built to help the resistance, has gone missing.
Farscape had, initially, one of the most evil Left Hanging endings ever. Knowing they were going to be canceled, the writers extricated the crew from the worst of the crap they were buried in, and set them on a planet to recuperate. This left some of the major arcs unfinished, but hey, we can deal with that, right? So there they are, recuperating. John proposes to Aeryn. It's a really touching moment. The crew are watching, all happy. Then, out of nowhere, the newly affianced couple get blown up by a random fighter craft.To Be Continued. On a show that's canceled. Luckily, there was enough fan pressure that a TV miniseries, the Peacekeeper Wars, was eventually made and tied up the remaining loose ends.
The Law & Order: Special Victims Unit episode "Harm" was rather bad with this. The detectives find that a woman may have been raped and murdered for helping refugees. They interview the widow of a man she helped, who claims her husband was murdered as well. They eventually book a doctor-slash-Torture Technician for the latter crime, but at her trial, one of the jurors faints, a mistrial is called, and the doctor's fate is left ambiguous. It appears to have been intended to make you "think" about liberty vs. security and all that jazz, were it not for two facts: first, the doctor is portrayed as an entirely negative character, and second, the plot of the initial murder is just dropped.
There was also another episode that left whether 1. The "victim" was raped by her teacher/date or 2. She was crying wolf entirely ambiguous. Said episode ended abruptly displaying the results of a survey whose participants thought 1. The man was guilty, 2. He was innocent or 3. There "wasn't enough" evidence to make a judgment on the facts displayed.
The season eleven episode "Savior" did this. A young prostitute goes into premature labor and her baby is put on life support. The mother then runs away, giving power of attorney to Olivia, effectively giving Olivia the choice of whether the baby lives or dies. The episode ends with the baby needing immediate brain surgery and the doctors hammering Olivia for a decision that she never gives. This turns into a case of What Happened to the Mouse?, as neither the baby nor the mother are ever seen or heard from again.
"Executive Producer Dick Wolf" are often claimed to be the most frustrating words in the English language, due to the many episodes of Law & Order, and the spin offs, that end without enough resolution or sometimes any resolution at all.
Due to being canceled by the second season, Pushing Daisies left a lot of stuff hanging, in spite of its sweet finale. Alfredo and Oscar were Put on a Bus by the end of the first season, and no one will ever find out where is Charles Charles or what the deal with Ned's father was.
Babylon 5 inverts this problem—the carefully planned five-season format of the original show wrapped up the entire premise of the universe at the end of the series, dooming future sequels to serious Sequelitis from the very beginning, as they seem like nothing but barely related afterthoughts to a story that has already been told. A more straight example is the Spin-Off sequel Crusade, which was Screwed by the Network only half a season in, leaving the Drakh plague story arc unresolved.
According to JMS's DVD commentary, the plague plot was to have been wrapped up in the second season. As with the original, the real plot was supposed to lead out of things that seemed like minor side details in the episodes that actually aired. The endgame for the Drakh war was handled in novels set more than a decade after the events of the full run of Crusade would have been over.
The '90s AMC series Remember WENN ended with an unresolved cliffhanger after the network's new management abruptly canceled the show.
Deadwood was canceled after three seasons, and had been intended to run longer. As the series was based on the real history of Deadwood in a macro sense, there was enough material and history left for at least a few more seasons. Due to the abrupt cancellation, several plotlines had to be hastily tied up, to no one's satisfaction.
Flash Forward left just about all plotlines open due to series cancellation.
Foyle's War was canceled and restarted multiple times, with each cancellation resulting in an attempt to tie up the series hastily, and then put everyone together again when the series resumed. Furthermore, the first cancellation resulted in the season then in production to skip several months of time in Real Life per episode. Prior to that season, each episode had a gap of days or weeks.
ALF ends with the eponymous alien surrounded by government agents facing certain vivisection. What happens to him after would later be covered in a TV movie.
My Name Is Earl was canceled at the end of the fourth season, which ended on a big cliffhanger and a To Be Continued. It's somewhat dealt with on Raising Hope where we find out that a "A local man who made list of good things to do finally finished it." Both shows are by the same creator, Greg Garcia.
iCarly has a few plot-points unresolved or not referred to, and they revolve around the shipping that is not the main focus for the show, and only comes up in a handful of episodes per season. There are differing ideas on what it meant, mostly around which side of the Creddie (Carly + Freddie) or Seddie (Sam + Freddie) shipping divide people stand on.
The first, is the end of "iThink They Kissed" where Carly asks Sam and Freddie (who shared a First Kiss, then hid it from Carly), how long it was, and if they enjoyed it. The episode ended before they answered the question.
The second, is at the end of "iSpeed Date", Sam walks in on Freddie and Carly sharing a slow romantic dance in each other's arms. She walks out with saying a word. Again, it's not been brought up again, so speculation abounds on her motives and feelings for Freddie (or Carly). Again, what people think is based around the shipping divide.
Finally, "iSaved Your Life", where Freddie saves Carly's life, they enter a relationship. Carly tries to say she loves Freddie, but Freddie still breaks up with her because Sam put it into his head that Carly just loved that he was heroic and she's just hero worshiping. Freddie says to Carly that if she wants to be his boyfriend in the future, he'd love to. Neither this nor any of the mentioned plot-points have been mentioned again, and a deliberate Cliff Hanger which adds even more questions, in the last episode of Season 4 means it'll be at least another 3 or 4 months before any could get possibly answered.
An episode of Walker, Texas Ranger had Cordell Walker rescue a girl who was trapped in a Christian cult camp, but with the last minutes of the episode devoted to Walker's rescue of Alex Cahill from the cult camp, it's unknown what has happened to the girl he was supposed to rescue. An even worse example is the movie "Trial by Fire" which ends with Alex being shot in the courthouse and lying near death. Supposedly, the producers were expecting CBS to offer them the opportunity to make further TV movies but low ratings (supposedly due to a football game preceding the movie running an hour longer than anticipated) and CBS shortly thereafter scrapping their Sunday night TV movie has made further TV movie's unlikely.
Probably due to extra seasons being planned but ultimately being cancelled by the BBC, season 3 of The House Of Eliott ended with an unresolved argument between Evie and Beatrice over the direction of the eponymous fashion house.
The final episode of Unnatural History ended with the cast in the Mongolian desert, when they hear a strange noise. Jasper and Maggie wonder what the noise was, and Henry suggests the area is "more than just dust and bones".
Criminal Minds: Suspect Behavior. The first season ends with a sadist/masochist killer team forcing team leader Cooper into a Sadistic Choice of killing the masochist or the sadist will kill one of his teammates. The choice is right down to the wire, cut to black, gunshot. But, sorry, show was cancelled the previous week, NO RESOLUTION FOR YOU!
So much for The Cape. Didn't help they reduced the seasons episodes from 13 to 10, and didn't even show the last episode on TV (you had to go to the website to watch it). So die hard fans will never know if the Big Bad will ever go to jail, Vince will ever clear his name, or his family will know that he is alive.
This Is Wonderland, a brilliant Canadian legal dramedy, ran for three seasons, and ended on three concurrent cliffhangers.
In the fourth series of Merlin Guinevere and (a fake) Lancelot are brainwashed with magic into kissing each other on the night before Gwen's wedding to Arthur, resulting in Gwen's exile and Lancelot's suicide. Although Guinevere is eventually welcomed back to Camelot by the end of the season and becomes its Queen through her marriage to Arthur, no one (including herself) ever finds out that she wasn't acting of her own volition when she cheated on Arthur, even though it would have only taken a simple conversation with Merlin (who knew that Lancelot was being controlled by Morgana) to clear up the issue (Gwen being smart enough to realize that the bracelet Lancelot gave her was probably the cause of her abrupt change in behaviour). Yet for whatever reason, the writers thought exonerating the pair of them wasn't worth any meaningful resolution, and the fact that Guinevere had to have found out at some point that Lancelot killed himself isn't ever addressed in any way.
The short-lived BBC soap opera Eldorado did this deliberately in the hope that viewers would demand to know what happened next and force an UnCancellation. It didn't work.
The Metal Gear saga surprisingly avoided this problem. The fourth game filled up pretty much all plot holes, with the exception of Fortune's Last Stand in the second game. According to Word of God however, it was originally supposed to be played straight, with Metal Gear Solid 2: Sons of Liberty being the last game chronologically in-universe; so whether or not Metal Gear counts as an example depends on whether one wants to invoke Fanon Discontinuity.
James Bond game Blood Stone ends with a Tomato Surprise that could been resolved in a new game, also the credits start with James Bond will return... But we know Bizarre is done for good and the next Bond game won't follow this one. You could however stretch things up and pretend somehow that Skyfall's villain was The Man Behind the Man.
Final Fantasy VII has references to "Techno-Soldiers" replacing human agents of Shinra early in the game, but is never elaborated on and is forgotten once Sephiroth is introduced into the game. It is believed by many fans that the original plot of the game would have revolved around a Turned Against Their Masters scenario, which was abandoned in favor of Sephiroth. The techno-soldiers themselves finally turned up in Dirge of Cerberus.
Early in Final Fantasy VIII, the heroes are assigned to help the city of Timber gain its independence from the Galbadian Republic. This storyline is quickly folded into the main plot, but the question of whether Timber becomes free in the end is never addressed. However, since the Timber mission was a low-paying and therefore low-importance goal, when the sorceress showed up and became a threat to Garden and the world itself, it sort of makes sense that no-one cared about that so much when the main plot kicked off.
Final Fantasy XII at some point completely forgets to resolve the Occuria situation. Yeah, the Sun-Cryst is destroyed, but why can't they make another? Why do they just seem to give up without any word? The sequel does not address this at all either. If the canceled sequel Fortress might have expanded on this issue is now a mystery for the ages.
At the very end of Baldur's Gate: Dark Alliance II, the player sees an ominous-looking cutscene showing the rise of a new Big Bad. This was intended to be a teaser for DA III, which was never made. Dungeon Keeper III suffered the same fate.
The Fallout series has the Followers of The Apocalypse in the first game and the very similiar tanker vagrants in the second game, both are due to bugs. Although Fallout: New Vegas does feature the former, so now we know their canon fate. The Fallout 3 expansion Broken Steel leaves Colonel Autumn's fate unresolved if you let him walk out of the purifier. Was he killed or captured, or did he leave the Wasteland entirely?
Dreamfall, sequel to The Longest Journey, tied up about two of the many different plot threads left hanging over the course of the game. It doesn't even tell you what happened to the main characters. To be fair, though, it is the middle part of a trilogy.
The Interactive Fiction version of The Hitchhiker's Guide to the Galaxy ends with Arthur setting foot on the planet Magrathea. The game promises a sequel, which of course never came. Also book 4 of the written version which was supposed to be a final book before the author decided to write a 5th one: we get the Earth back (destroyed at the beginning of book 1) without any explanation and most of the cast is largely forgotten. Book five ties up most of the loose ends and replaces the previous resolution with a Downer Ending.
Gears of War 2 is paced from beginning to end by dropping the latest subplot and introducing a new one.
Gears of War 3, which Word of God confirms is definitely the last game in the series, does do a good job of wrapping up each characters' personal plot arcs and finally explains Imulsion and the Lambent, but still leaves unaddressed the Sires/New Hope Facility sub-plot from the second game, the true nature of the Locust (if any), as well as the question of who or what Queen Myrrah really was.
This was solved with old forum posts and the tie-in comic The Slab. Though, Adam Fenix's disk is never brought up again.
No More Heroes really ends with just Travis Touchdown and Henry striking each other by the Santa Destroy flag. This leaves many things to consider, such as what the UAA really is, who Travis's parents were, who Darkstar was, and, of course, if the events affected the video game's real world or if it really was Silvia's daughter Jeane day dreaming about a picture. Some, but not all, of these questions are answered in No More Heroes 2: Desperate Struggle.
The original Japanese release of MOTHER ended like this. After sending Giegue packing, Ninten and his party start to walk away from the battle field... and then the scene pauses, the credits roll, and you're left with a "To be continued..." for your troubles. The unreleased American localization averted this, adding an epilogue that gave a proper resolution for all the characters in the game; this was kept in the game when Mother 1 + 2 was released on the Game Boy Advance years later.
F.E.A.R. has one of your allies, Spen Jankowski, in a different area of the same operation as you. He disappears on the site. Eventually, the support guy stops mentioning him, and he never comes up again. You finally find his body in Project Origin.
Persona 4 seems to be doing this deliberately; while the main plot is resolved well enough, there's a fair few details, large and small, left completely dangling. The largest one by far is that, despite defeating the thing that was shrouding the "Other World" in fog, the TV-based portals to the other side still function, in stark contrast to the cessation of the Dark Hour after the Big Bad of Persona 3 is defeated. The cast themselves briefly note that this has potential issues for the future. It's quite possible that Atlus has simply left hooks in for another sequel.
Persona 2 At the end of Eternal Punishment, we see Maya recognize Tatsuya. . .but walk past, because she can't talk to him.
The Viewtiful Joe games were planned to be a trilogy, with a character in the first game even outright telling the protagonist that he'd have two more fights for justice on his hands in the future. This made it all the more incredible and infuriating when come the end of the second game, Joe's father turns out to be an evil villain, powered by a mysterious black V-Watch he acquired somehow, the superpowers and other craziness of the movies have somehow made their way into the real world, and a fortress of evil rises out of the earth in the distance, signalling the final and oh-so-presumably-epic battle and the answers to all our questions... but then, OH DEAR, the studio shut down, and we never got closure on any of that, or the overall story of the games.
Mega Man Legends 2 ended with Mega Man Volnutt stuck on the moon, and Roll Caskett and Tron Bonne working together to rescue him. This was left to hang for an entire decade, and with the cancellation of Mega Man Legends 3, it seems it's not going to be resolved any time soon.
Hydrophobia ends abruptly without dealing with a lot of the plot points: the credits roll just as the heroes encounter a new danger, you never find out about the person you're trying to save, the full details of what the heck was going on is never revealed, as is info about the Big Bad and the organization behind the plot. You can get the bare bones from the various collectible documents spread about the game, but that still only gets you the lead-in to a possible reveal, which never comes.
The original Dead Rising left off with Frank and Isabella escaping the mall and defeating an insane military general. However, Carlito spread infected orphans all over the country and Frank himself is infected. It also isn't mentioned as to whether or not Otis escaped with the survivors successfully (and there's the fact that a few of the survivors were infected anyway). The sequel pretty much confirms that the country-wide infection has more-or-less succeeded. Case West reveals that Frank and Isabella have survived, Frank is suppressing his zombification via Zombrex, and the two are hard at work to get to the bottom of the whole mess.
Mio's fate in the canon ending of Fatal Frame III is left deliberately ambiguous, but given how these games tend to go, she probably didn't survive. In the fourth game, the question of whether or not Misaki lived is also left unanswered.
The bizarre ending to Monkey Island 2: LeChuck's Revenge has never been resolved — by the time The Curse of Monkey Island came around, Ron Gilbert was no longer working on the series, and the devs for that game decided to Hand Wave it away rather than come up with their own explanation. A Word of God answer to what happened doesn't seem likely, either, as Ron Gilbert seems unwilling to share this (or the true Secret of Monkey Island — they appear to be connected) unless he can make a game out of it.
Return To Krondor had an ending that was clearly intended as a Sequel Hook. Let's see... Sidi is still alive and active, and puts the amulet back together. He intends to release the Dark God into Midkemia and the amulet is clearly a part of his plan. Meanwhile, there is the matter of the Crawler still alive and and active...somewhere. A sequel has never been made.
Touhou's Hisoutensoku has three story arcs centered around three heroines searching for the owner of a giant shadow/silhouette. Only Sanae's arc has sufficient closure - that she had, in fact, found her "giant." Cirno gets sidetracked twice by Marisa and ends up fighting one of Alice's experimental spell cards, pegging it as her Daidarabotchi. Meiling gets it worse: either she dreamed the whole thing, or she did see something but never even got to her first stage battle.
Legacy of Kain has lain dormant since 2003's Defiance, with the Pillars of Nosgoth still destroyed and the future of the world uncertain. Crystal Dynamics' shift to the Tomb Raider series, the series' creator's move to Naughty Dog, and the death of Tony Jay make the chances of a resolution appear unlikely.
The details of Nero's background in Devil May Cry 4 remain very blurry. We still don't know whose son he is (claims that he's Vergil's remain Fanon), when or how he got his Red Right Hand, or how much Dante knows about him… and apparently we will never know since Capcom decided to let Ninja Theory make a "rebirth" of the series rather than a sequel.
This trope is one of the many complaints regarding Mass Effect 3's ending - no matter what you did throughout the entirety of the series, you receive one of three endings with no closure outside of the fate of the Reapers. "Side" missions such as using/sabotaging the genophage cure, whether the Quarians or Geth (or both) were saved, and the fate of the Rachni have no narrative consequence outside of War Assets, and the fate of your squadmates is left completely unknown.
Another common complaint is that Shepard only can survive in the Red Ending if you do everything right, but even then, its a split second of his/her torso taking a laboured breath in a pile of rubble. Smash Cut to credits, leaving his/her fate completely unknown.
The ending Killzone 3 abruptly smashes you in the face with the credits immediately after the climax. There's a short stinger scene in the middle of the credits which answers one minor question but that's as far as it goes. As far as the player knows: 1. The Helghast fleet has been mostly destroyed. 2. The nuking of Stahl's flagship on the surface of Helghan triggered a massive Petrusite chain reaction which appeared to wipe out all life on the surface, with the obvious exception of Stahl and a few of his soldiers as seen in Stinger mentioned. 3. Considering how powerful the Helghast weapons have become, and how some of their ships warped to Earth, it's possible the attack on Earth has already begun. 4. The ISA forces left at Helghan were either destroyed by the petrusite reaction or stuck flying around the planet in fighters incapable of warp travel, so they probably have no way to get to Earth.
Freedom Force vs. The Third Reich ends with Alchemiss/Entropy meeting the living embodiment of Energy X who implies that she still has a job to do. The third game in the series, which was intended to cover modern and Iron Age comics, was never greenlit.
Golden Sun: The Lost Age resolved most of it's major plot points, but left a few smaller plots open -Namely Anemos, Sheba's origins, and the ultimate fate of Alex. The sequel, Golden Sun: Dark Dawn, revealed that Alex survived but otherwise did not address any of these points, and even introduced a few more: The quest Takeru is on, what happened to Felix, the Tuaparang Emperor, and especially The Psynergy Vortexes, particularly the huge one that has manifested over Matthew's home. Oh, and Alex is still The Unfought.
What happened to Alis(a) after Phantasy Star Gaiden, and what exactly was the threat she was returning to Algol to face? At the time it was made, she was supposed to return as the protagonist of Phantasy Star IV and this game was the set-up for that, but when they ended up going with a different plot for that game, they just left this unresolved.
Star Wars: The Force Unleashed II ends with Darth Vader captured by the Rebellion and the Rebel fleet makes the jump into hyperspace, little knowing that Boba Fett is following them. Unfortunately, the lack of sales, combined with LucasArts closing in 2013, suggests that it's not going to be resolved any time soon.
Epic Mickey 2: The Power of Two ends with Mickey and Oswald saving the wasteland and defeating the Mad Doctor. The post credits scene features all the Petes of the wasteland getting together and plotting something while holding Gremlin Prescott hostage. However, due to the game's poor sales which led to the closing of Junction Point Studios, a third game is highly unlikely.
Assassin's Creed III left three unsolved mysteries: Connor's ultimate fate, Juno's true plans for the world, and that mysterious voice at the very end. Only the last was ever resolved (an Abstergo tech guiding one of the company's Animus users). In Black Flag, Connor is only briefly mentioned with no new information, and Juno for some reason is helplessly trapped in Abstergo's network, only saying that "perhaps the Vault was opened too soon".
The Epic story of Shenmue ended on (perhaps) the 4th, of it's planned 11 chapters, when it was cut-short after it's sequel. For a multitude of reasons but mainly because Sega could no longer afford to make the games any more. Leaving not only the whole driving revenge plotline hanging but introducing us to new characters that would apparently answer many questions about the mysterious Macguffins, character motivations and if the world was Maybemagic Maybe Mundane.
An old joke around this trope; a man living in an apartment building has fallen into the habit of dropping his shoes on the floor when he gets home and takes them off. This causes his downstairs neighbour to complain about the frequent noise. One evening, the man comes home, takes off one of his shoes and drops it, but then remembers his neighbour's complaint and places the other one quietly on the floor beside the first shoe. Hours later, he's going about his thing when he suddenly hears an anguished cry from downstairs: "For God's sake, drop the other shoe!" Ba-dum-tish.
Nine Hours, Nine Persons, Nine Doors: Though you may not know it: The true ending seems to imply Junpei finds Akane, right? Wrong. He doesn't. He spends the rest of his life looking for her, and she goes off on some huge, grand scheme that the director only mentioned in passing.
The sequel Virtue's Last Reward is even worse, the entire game being training set up by Akane and Future!Sigma to develop Sigma and Phi's esper abilities to send them to the past and prevent the Radical-6 outbreak. Naturally, the game ends with them jumping back in time, and neither epilogue takes place when any of the participants willing to divulge information have yet been in the Mars mission test site.
Pick a comic listed on Orphaned Series. Nearly all of them stopped in the middle of ongoing storylines.
In Sonichu, they had just reached big final showdown between Mary Lee Walsh, after her defeat they release the most powerful evil being in the comic, Count Graduon, and then... the creator angrily announces he hates his greedy fanbase and swears off the internet, and abandons his website.
As Tessa Stone hasn't been heard from in months, Hanna Is Not a Boy's Name is like this, at least for now. The comic stopped updating when every major character was in mortal peril. Zombie, Hanna, and Toni are locked with a questionably sane Ples (who just whipped out a gun and fired it), and Conrad's trapped with Lamont, Worth, and Adelaide trying to get to Hanna and escape Abner. Oh, and Veser ran out a while ago, and we never found out where he went.
RPG World, the heroes have finally made it to Galgarion and invade his headquarters. Everyone's about to throw down and...it just stopped. A combination of creator, Ian J, going into animation and backlash from from his fans resulted in him quitting the webcomic entirely, leaving the story unfinished after a seven year run (2000-2007). What's more there was a subplot going on that Rika's robots had stumbled upon a time machine meaning their could possibly be a way to prevent her from being killed. But the series stopped before it can go anywhere.
The X-Entertainment 2008 Advent Calendar's last update was the 22nd... right before the traditional final battle. Great.
Little White Lie featured a major cliffhanger...and then the creators lost their money and most of the people moved and scattered around the country.
The ending to Candle Cove is a mixture of this and The Reveal. The last person discussing the long-forgotten children's show explains that Candle Cove didn't exist at all, and that anyone who watched the show was just watching static for half an hour. It turns into Paranoia Fuel when the participants in the story remember some profoundly scary things from the show.
The BIONICLE web-serials ceased being updated long ago, for the simple reason that the writer didn't have time to continue doing them, leaving about a dozen or so different plot-lines hanging, some of them in their very early stages. And as these were the sole story-sources, after the cancellation of the toys, comics, books and movies, this effectively means the entire franchise ended this way.
Chris Niosi may NEVER stop hearing backlash from fans about how he ended TvTomeAdventures. Granted, he WAS planning on creating a fourth season, but after he went to college, he had less time for the series and the show was cancelled. Still, most of the loose ends can be answered by scrounging Niosi's deviantART page. It's a shame, too, since season 4 would have been awesome.
Because a Creator Breakdown torched it when it didn't have to be, Demo Reel left a few things unanswered, most annoying being Donnie's failed marriage. What did he do to make it die? Did she know about his past? How did they meet? We'll never find out.
In We Are All Pokémon Trainers the PMD-R arc abruptly ended by leaking into Kanjoh-2 due to most of the playerbase losing motivation, though there have been attempts to properly fill the gaps and close the loose ends.
As Told by Ginger, "Wicked Game" (about Dodie's attempts to break her best friend Ginger's relationship with Darren). This is brought up again in a later episode as an offhand comment, so it's assumed that Ginger, Dodie, and Macy patched things up off screen and deal with it by never speaking about it again.
The Hey Arnold! episodes "The High Life" (Gerald has no money to buy rollerblades... and at the end, because of the phone line etc. he got to sell his watches to get the money, he still has no money to buy rollerblades) and "Arnold Betrays Iggy" (Arnold is mad at Iggy for forcing him to wear embarassing clothes in public and refuses to forgive him, mirroring an earlier scene where Iggy refused to forgive Arnold for exposing his secret, and he didn't even do so). And we never do find out what happened to his parents. The episode that featured their flashback ended with Arnold finding a map and rushing to his Grandpa about it. This was actually intended to be the lead-in to a second Hey Arnold! film which would resolve this question, but Craig Bartlett's departure from the show coupled with the low gross of the first film led the series to a premature end before this could be resolved. This movie also would have resolved the cliff-hanger over how Arnold would have responded when Helga, at the end of the first film, confessed her true feelings to him.
The Weekenders, "Croquembouche" (about Carver in a food essay contest: Tino does his usual end-of-episode Aesop routine while Carver presents his essay on a French cake, which gives the ep its title, and the ep ends with people applauding Carver's speech, without showing if he won or not)
All Grown Up!, "Izzy or Isn't He?" (when the episode ends, you realize they never mention the result of the election that forms a major part of this episode's storyline).
ReBoot started the "Daemon" storyline during the third season... only to find out that they weren't getting a fourth season. That one was left hanging for years until the TV movie Daemon Rising (which used "It's About Time!" as part of the advertising). Alas, ReBoot ended on another cliffhanger not long after. Other three movies entered into production, and a webcomic is being done on the official ReBoot website to bridge the storyline gap between the old and new materal. But then it all ends in another cliffhanger.
In the BattleTech animated series, the final two-part episode pits the main characters against the bad guys in a Trial of Possession for the main characters' home planet of Sommerset. When it finally breaks down to a hand to hand fight between the bad guy and the hero, it ends with the hero winning the planet... but not the people, including the hero's brother, who were all spirited off the planet. The series was not continued. However, in a novel set in the same universe, the main character shows up nearly 15 years later. He is presented with the temptation to make an unauthorized attack to retake Sommerset, meaning that the Trial of Possession (much like the rest of the series) had little or no lasting effect on the larger Battletech universe.
A Tie-in comic called The Search finally answers the question.
The 1990's cartoon Mummies Alive! ended with one of the minor characters (who'd been searching for proof of there existance) listening to fellow believers on his own radio show in what boils down into an extended Clip Show. The series was supposed to be picked up for another season, but this fell through due to low ratings.
Spider-Man Unlimited ends with Venom and Carnage-esque symbiotes being spread across the world. A book on Spider-Man mentioned that had the second season happened, Spidey and the resistance would save the day. The class system on the Second Earth would be abolished and Spidey would return to his own planet. Looks like they were going to go back to basics.
Robotix ended with Nemesis, who had been presumed deceased, still alive in space. If the episodes had been picked up as a full series, he and the other Terrakors would have most likely returned to Skalorr to get revenge on the Protectons.
Lampshaded again in a different episode, where a supermarket bagging boy strike tangentially led to the family going on an African safari. As the family drifts down a dangerous river on a very makeshift raft, Homer wonders if the strike back home is over. His rant is cut short when they careen over Victoria Falls. After they survive this, they follow a monkey in hopes of finding their way back to civilization... as Homer continues to rant on why those bag boys don't deserve anything.
Interestingly inverted in one episode where Homer, going crazy with Throwing Down the Gauntlet to scare everyone, ends up offending someone who accepts his challenge and he and the family are forced to flee the house while he's waiting on the lawn. At the end of the episode when they return from a failed farming venture, Homer finds his opponent still waiting for him and they end up dueling after all.
The episode, "Missionary Impossible", with Homer on an island, where he and a girl are at the top of a tower, which is about to fall into a lava flow with them in it. The episode's plot ends there as they get interrupted by a callback to the telethon fundraiser Homer ran away from in Springfield (and a joke about Rupert Murdoch being so greedy that the show's revenue from commercials and merchandise isn't enough).
Duckman's last episode, "Four Weddings Inconceivable", ends with the titular character about to remarry—until his first wife, who has been presumed dead for the entire series, shows up. According to the series' entry at Wikipedia, "Writer Michael Markowitz noted (in Sep 1998): "We never formally planned Part II... and I'll never tell what I personally had in mind. I'm hoping to leave it to my heirs, for the inevitable day when Duckman is revived by future generations. Ah, the Spandex suits they'll wear, the hovercrafts they'll fly!"
The French and Vietnmese dubs of Beast Wars both ended after the second series, leaving fans with a massive, unresolved cliffhanger.
One could argue that the series itself ended up like this. While the main plotline was largely resolved, the nature of the Vok aliens (who instigated some of the series' biggest episodes) and the origins of Tarantulas were not explained. This was a result of the series being rushed to completion so that the sequelBeast Machines could begin. Simon Furman eventually revealed that the season 3 finale was originally to be a 3-parter that would end on a cliffhanger, leading in to season 4 - this Season Finale was to explain Taratulas' origins in detail and his motivations for hating the Vok (and would likely have resulted in Tigerhawk gaining a bigger role as well instead of being thrown conveniently under the bridge). The plot was rushed, and unexplained aspects of the story were left to the comic continuation.
Roughnecks: Starship Troopers Chronicles details in its final season the invasion of Earth. Anyone Can Die is established fairly quickly, the queen is on Earth, and the whole thing seems to be heading to a massive climax. Then Sony cut the funding three episodes from the end.
The ending of the second season of Legion of Super Heroes revealed that Brainiac was still alive and well ("Evil does not die; it evolves!") But the show didn't get a third season.
Sonic Sat AM: The final episode of Season 2, "The Doomsday Project" ends with Robotnik's sinister plot to launch doomsday pods all over Mobius ruined, and the city of Robotropolis can return to Mobotropolis as Dr. Robotnik is left to an unknown fate. Sonic and Sally realize their emotions for each other and kiss. But that's not it—Snively tells Sonic not to be so happy as it's now his turn, and a mysterious red-eyed figure (revealed in a later interview to be Ixis Naugus) laughs threateningly. Sadly, the show was canceled after Disney took over ABC.
The Spectacular Spider-Man. Did you enjoy seeing all your favorite major and minor characters from classic Spider-Man history and can't wait to see how they develop? Want to know what kinds of juicy drama will take place now that Gwen hotted up, and is stuck with a seemingly revenge-bent Harry Osbourn? Or just how in the world his father survived that nasty explosion and what he's planning next? Does Peter end up with MJ or Gwen? Does Gwen even live? Well tough luck bub, it got shitcanned.
Every Spider-Man based animated series so far was Left Hanging, in fact. note Starting from the 90's show at least. To wit...
Spider-Man Unlimited- Ended with with Venon and Carnage carrying out their plan and raining down symboites on Counter-Earth.
Spider-Man: The New Animated Series: Peter defeats the Gaines Twins, but his friends Indy, is injured thanks to them tricking Peter into attacking her. Harry, who was starting to come around that Spider-Man might not be so bad, goes back to hating him and Peter ultimately quits being Spider-Man and throws the costume into the Hudson River.
The second season of The Avengers: Earth's Mightiest Heroes has a subplot regarding the return of Surtur, the fire demon, and the possibility that he will cause Ragnarok to destroy the Earth. In two episodes, Surtur wipes out the dwarves of Nidavellir, enslaves Amora the Enchantress, and slaughters Korbinite aliens. A bonus episode on one of the show's DVDs features copious buildup to a confrontation between the Avengers and Surtur. However, since the show didn't receive a third season, this confrontation ends up never taking place.
The Oh Yeah! Cartoons short "Planet Kate" ends on a cliffhanger. We never find out what the alien dogs' mission is or why the map Kate made for a homework assignment happened to fit perfectly as the other half of a real map.
Likewise The Modifyers ended with Agent Xero and Mole managing to reclaim the All-Seeing Eye but she loses her communication ring which the villains, who only know her by her other persona, Lacy Shadows, find on the floor.
Western Animation/Tron:Uprising seems to have ended this way. This means that we never find out how Clu managed to capture Tron and turn him into Rinzler, if Mara and Zed ever found out that Beck was the Renegade, if Beck ever proved to them that he did not kill Abel, if Paige ever underwent a Heel-Face Turn and joined Beck's side, and whatever happened to the uprising, Argon City, and virtually every single character in this show with the exceptions of Tron, Clu, Quorra, and Flynn.
Green Lantern: The Animated Series ends with Aya coming to her senses when she accidentally hurts Razer, then reversing her attempt to rewrite the universe and destroying the remaining Manhunter army with a virus that also affects herself, then she disappears into the aether. At the end Razer is convinced that she's still out there somewhere in space and goes off to find her, his hope for doing so causing a Blue Lantern ring to follow him.
Star Wars: The Clone Wars Ahsoka leaving the Jedi Order, but so many plot resolves (i.e. What happens on Mandalore, what did Darth Sideous do to Darth Maul, "did Palpatine really clone the Zillo Beast?", and "How do Poggle the Lesser and Wat Tambor escape from prison to be in Revenge of the Sith?) were left hanging after the show got axed after Season Five ended.
Sym-Bionic Titan was cancelled before any plot points could be resolved. Some people say the cancellation was due to lack of toy sales.
While X-Men had a definitive finale in the episode "Graduation Day", the series had few unresolved plotlines: Friends Of Humanity organization is still out there and is implied to be even more powerful, an episode with Cannon suggested that shady Government agents will try to manipulate mutants to destroy each other and most importantly the Apocalypse had returned to plan more evil.
Zorro Generation Z ends on the corrupt mayor losing his reelection campaign to a much more competent crime boss who also coopted most of his former henchmen in the process. And to stand against this new, greater better, Zorro and Scarlet Whip reveal their secret identities to each other. Because a second season never happened, the increased threat to the city and our hero was never resolved. Although strangely, the intro sequence that was made for the second season looks as if they would've gone back on this, showing the former villain a lot as if he'd be the main antagonist again.
Gary And Mike ended with the duo framed for murder and cornered on a incomplete bridge by the police, Gary's father (whose looking to punish him for destroying the SUV, though that was Mike's doing) and Officer Dick (whose after Mike for sleeping with his daughter and costing him a promotion). With nothing left to lose, the duo decide to recreate Thelma & Louise and drive off the bridge. The screen whitens as they do so and were given a "To Be Continued" but the show was cancelled after that.