Kid Colt (2009): In the first issue a United States Marshal, Samuel Gemmell, unexpectedly drops in on the crooked sheriff McGreeley (the Big Bad of the series), telling him that the district judge is taking an interest in the Kid Colt case, and will soon be visiting McGreeley. Colt himself later pays a visit to the Marshal, holding him at gunpoint while explaining that he was framed and is trying to clear his name. Gemmell listens to Colt's explanation, but never appears again, and the judge's visit plays no part in the the story.
The Dark Phoenix Saga: During the X-Men's battle against the Imperial Guard, Wolverine indirectly instigates a battle between a Skrull named Raksor and a Kree warrior named Bel-Dann. In the original ending, the narrator would say they killed themselves off-panel, but in the released issue, their fate is unaddressed. The 1984 Fantastic Four annual would later pick up their story where it left off.
In 1993, there was Scott Summers' long-lost second brother (mentioned but never found, and soon forgotten by everybody) and Wolverine's recurring vision of a tree with radiators growing out of it (never explained). The "3rd Summers brother" plot was finally resolved 13 years later in X-Men: Deadly Genesis which revealed a new character Vulcan to be born in Shi'ar-member of forgotten second X-Men team-Ax-Crazy Gabriel Summers. Long before that there was a common belief that it was Adam X-Treme, son of Shi'ar emperor D'Ken. So it's more a Brick Joke.
This was resolved, Adam Neramani (X-Treme) is Scott and Alex's Half-Brother, but not a Summers, as he is the son of their mother and D'Ken, and not the child of Christopher Summers (Corsair).
Flipside of Adam-X, Corsair's girlfriend among the Starjammers, Hepzibah, was pregnant with his child when Corsair was killed by Vulcan, while Hepzibah has been seen several times since her escaping to Earth and bonding with Warpath, if she did or didn't have the baby and its whereabouts if she did carry it to term have been left unknown.
Apocalypse almost ended up being made into the third Summers sibling but thankfully editorial squashed the plotline. On the other side, as part of the "X-Men: The End" trilogy, Chris Claremont made Gambit the third sibling of sorts in said alternate reality storyline, but with the twist being made that Gambit was a clone-son of Sinister and that Sinister spliced in Cyclops' DNA (his X-gene, specifically) as part of his plot to use Gambit as a host body to kill Apocalypse. Claremont's storyline itself was largely seen as a "Take That!" against Wizard which heavily pressed in the pages of its magazine for Gambit to be revealed as the third brother (using the dubious logic that they were both 20-something white American males with brown hair, red eyes, and explosive energy powers). It's also a reference to Claremont's original plan for Gambit to be revealed as another persona of Sinister's intended true identity as an immortal mutant child, the child's version of a "roguish hero" archetype to Sinister's "archvillain."
In a 1989 issue of Uncanny X-Men, Chris Claremont introduced a group of Badass Normal ex-SHIELD agents-turned-mercenaries called the "Harriers" led by a man named Hardcase, an old friend of Wolverine's. They were made to look like a potential group of recurring characters even going so far as having a "created by" credit caption in the splash page with Claremont and penciler Marc Silvestri. So far, though they have only appeared in that one issue.
Numerous X-Men titles introduce characters who are prominent for a while then generally disappear. For example, X-treme X-Men introduces Lifeguard and Slipstream, who are important members of the team until Slipstream goes nuts and runs away. Lifeguard leaves with Thunderbird to find him, putting them on the proverbial bus. Lifeguard resurfaced in a single panel in the early 00s "Excalibur" series; Thunderbird and Slipstream didn't.
It was later revealed in an official Handbook that Thunderbird and Lifeguard retained their powers after the Decimation while Slipstream was depowered. Lifeguard was also mentioned to have joined the Xmen in San Francisco on Utopia, but no word on what happened to her brother or her boyfriend.
The 1987 Mephisto Vs.. miniseries concluded with the revelation that Mephisto had acquired Rogue's soul. This has never been mentioned again.
In the 1960s Captain America (Tales of Suspense) series, "Midnight In Greymoor Castle!" is the final World War II period story in a run of Whole Episode Flashback tales, and had Rogers leave his unit in the field to help Bucky (who has been captured and held in a castle in Britain). Although Cap later saves his unit by smashing their attackers with a V-2 missile, as Rogers he's suspected of desertion and still in serious trouble. In the next story, the series returns to the present day with Cap telling that story and the Scarlet Witch asking if he got in trouble - and Cap simply explains that US military intelligence covered for him.
During JMS' run on The Amazing Spider-Man (J. Michael Straczynski), a villain called Tracer was introduced, claiming to be a machine god, kinda like Thor. When Peter defeated him, he said he would step off the path of godhood for awhile. He hasn't been seen since.
Super-villain FACADE is rather infamous for the non-resolution of his storyline. He was introduced in a 4-part murder mystery story in 1994, where someone murders long-time supporting cast member Lance Bannon within the offices of the Daily Bugle. There are clues pointing to multiple suspects, including other staff members of the Bugle and their families. The story ended without Spidey or anyone else finding who the villain is, and promised to be continued in a sequel. But the next issues featured an unrelated gang war, then the Clone Saga begun, and no sequel was published. Finally in a 2012 story, FACADE offers to reveal his identity to Spidey, but Spidey had bigger fish to fry at the moment and basically ignored the villain.
Though most people just know her as a longtime anti-heroine today, Spider-Man villainess the Black Cat was introduced in a whole convoluted storyline. It involved J. Jonah Jameson, usually unseen in the story proper, slowly losing his mind, Robbie Robertson, his right-hand man, taking on his boss's characteristics, a whole bunch of other threads slowly coming together, and the Black Cat getting in the middle of all this. All plot threads were abruptly dropped when the writer left the series suddenly, causing the story to be hastily rewritten so that the Black Cat is shown to be stealing items out of a twisted love for Spider-Man, who she sees as a substitute father figure. It was eventually revealed many years later that if the storyline had been concluded as it was originally intended to, J. Jonah Jameson, in a deranged fit, would have gunned down the Black Cat.
Another Spider-Man example involves a new group of supervillains introduced in 1993, the New Enforcers. While investigating a failed assassination attempt, Spidey encounters a group of second-string supervillains such as Plantman and the Vanisher, who are claiming to be the New Enforcers. He manages to defeat and capture all of them, unaware that these were only the foot soldiers/henchmen of the New Enforcers. The actual leaders are revealed to the reader to include some more prominent characters, such as Madame Menace/Sunset Bain and Fixer/Norbert Ebersol. They are hinting of further plans to take over Kingpin's criminal empire, and they have a connection to a new, mysterious character "Mr. Graycrest", who seems to own their headquarters and has past connections to Roxxon (Marvel's everpresent evil corporation.) The story ends in a cliff-hanger, and the team was never seen again. Most of the characters resurfaced, but Graycrest has since only been name-dropped in a story involving Punisher and Sabretooth.
Dan Jurgens began his run on The Mighty Thor with the Thunder God once again forced to assume a mortal guise, this time occupying the body of an EMT called Jake Olson, who was killed during a battle between Thor and the Destroyer. Jake's partner, Demitrius Collins, is seen stealing drugs from their hospital and selling them on the street, and plots to pin the blame on Jake. Sure enough, the epilogue to the first story arc (in which Thor frees the missing Asgardians from their captivity at the hands of the Dark Gods) features detectives tracing the stolen drugs to Jake and finding them planted in his appartment, leading the reader to assume that Demitrius has made his move. However, in the very next issue, Jake is confronted by the police, discovering that Demitrius was actually an undercover detective all along and had determined that Jake had been a drug dealer before Thor assumed his identity, and the reader learns that the real Jake Olson was a criminal when the long-missing Loki returns to recover his tormented soul from Mephisto's realm and place it in a new body as part of a new scheme against Thor. This was a genuinely clever twist... but it still failed to explain why Demitrius was selling drugs, prompting many fans to assume that Jurgens just forgot the earlier plot point!
In Marvel's Laff-A-Lympics book, Scooby-Dum (Scooby Doobies) and Sooey Pig (Really Rottens) were left out. Not that the Scoobies and Rottens ever noticed.
If Gerry Conway hadn't dropped Gwen Stacy off a bridge, this would have been the most famous legacy of his 70s tenure in the Marvel Bullpen. He had an infuriating tendency toward using Sequel Hooks and dangling plot threads as resolutions that rendered a lot of his stories unsatisfying. A prime example was the eight-issue arc climaxing in The Mighty Thor #203 with the creation of the Young Gods. Every sign pointed toward these characters immediately playing a massive role in the title; instead, they were shelved for seven years.
Fred's fate in the Bad Future of Zombies Christmas Carol is unknown, though given the emphasis on Scrooge's redemption it's likely that even Fred's kindness wasn't enough to stop the rising tide of undead.
In Marvel Zombies the original zombie was the Sentry who eventually reappears in a Stable Time Loop. We still never find out what happens to him. He might have been killed when the zombies that ate the Silver Surfer killed the other ones, but he wasn't even seen in those comics. Marvel Zombies 2 also had Colonel America come back in the Black Panther's son's body. In Marvel Zombies Return, we find out the other characters were killed, but he's still not mentioned.
Daredevil vol. 1 featured a number of them, often due to changing writers.:
In 1975, Matt Murdock angers Corrupt Corporate Executive Ernest Rockworth by refusing to work for him. There are scenes where Rockworth is plotting some kind of revenge, but the subplot is soon dropped. Rockworth has not been seen since.
In 1975, Daredevil faces Well-Intentioned Extremist Torpedo/Michael Stivak, unaware that Stivak's crimes are part of a personal crusade against a criminal syndicate. Soon after, Stivak gets killed in action and the Torpedo identity is taken over by Brock Jones. Daredevil can't see any difference between the two men, and continues to pursue Jones for his predecessor's crimes. The storyline was dropped after a number of issues, and Torpedo/Jones turned up afterwards in other Marvel titles. With everyone treating him as just another superhero.
This is eventually resolved in some issues of Captain America and Hulk that reveal that Stivak's brother was part of a criminal syndicate called the Corporation that had appeared in those titles, though Daredevil and the Torpedo don't show up.
In 1984, Matt Murdock is hired to defend low-level criminal Stewart Schmidt in his upcoming trial. To their mutual surprise, Stewart turns up to be "Stymie", the neighborhood kid who bullied "Mattie" Murdock for most of their shared childhood and teenage years. (As seen in flashbacks from older stories). They have barely interacted in the last 20 years. During the story, Stewart gives cryptic answers over why he used to target Mattie over any other kid, and it is increasingly unclear whether Matt is acting as a professional or seeking some kind of revenge. Near the finale, Matt drops the case before it even gets to court, and Stewart tries to provoke a fist fight between them. The story has an ambiguous ending, and has seen no resolution since then.
In 1993, super-villain Mr. Fear/Alan Fagan learns that he has an illegitimate daughter, Shock/Ariel Tremmore. The girl was trained to hate her father with a passion, and has similar powers to him, along with werewolf-like features and powers which he never had. She arranges a near-fatal attack on her imprisoned father, and then tries to eliminate his enemy Daredevil, in an effort to be a better villain than her dad. At the end of the storyline, Daredevil mistakingly thinks that he removed Shock's powers (while he only suppressed them for less than an hour), Shock is back on the warpath, and Fagan is plotting revenge from his hospital bed. Shock hasn't been seen since, and Fagan has rarely appeared.
There is a rather peculiar one from the 1990s series of Dr. Strange. In 1993, Strange was at war with the demoness Lilith and her children, the Lilin. When all the demons are either killed or exiled to other worlds, Strange discovers that a young female demon, Sister Nil, was stranded on Earth. She feels very alone and sad, and asks for him to keep her company. Strange binds her to himself, as something of a witch's familiar, and treats her as his ward. She was a supporting character until 1995, seemed to develop romantic feelings for Strange (including acting jealous whenever Strange contacted Clea), and attempted to befriend the demon Nightmare, because she recognized that he too felt desperately alone. Following the end of that series, Nil has never been seen or mentioned.
Conan the Barbarian: The Prince in the Dark Horse print served as an audience surrogate whom the story was narrated to the readers through him. He hasn't appeared again in any recent comics published since Conan the Cimmerian.
