At the end of The Avengers #16, the departing Iron Man told the new Avengers that they should track down The Incredible Hulk and convince him to rejoin the team, as they currently lacked the raw strength of the original lineupnote As Iron Man, Thor, and Giant-Man all took a collective leave of absence.. Despite the end of the issue promising "The Search for the Hulk!" the following month, issue #17 instead dealt with the Avengers facing the Fantastic Four villain Mole Man, with the Hulk subplot essentially being abandoned within the first few pages.
Near the end of Roger Stern's run, minor Alpha Flight character Dan Smallwood saw the Avengers on TV, and was shocked to learn his childhood sweetheart Marrina was now a member of the team. According to Stern, this was going to lead to Dan heading to New York and ending up in a Love Triangle with Marrina and Namor. Unfortunately, Stern was removed from the book by Executive Meddling, and Walt Simonson ended up killing off Marrina and writing Namor out of the series in his very first arc.
Simonson himself would soon fall victim to this. During the Inferno (1988) crossover, Mister Fantastic and the Invisible Woman helped form a new team of Avengers in a manner similar to the formation of the original team way back in the first issue many years ago. Despite clearly being positioned as major players in the new Avengers, they were quickly written out of the book once Inferno ended. It turns out that this is because Simonson had been given permission to add the two to the Avengers, only for editorial to reverse this decision and demand they be sent back to the Fantastic Four instead.
Steve Englehart had Mantis show up an in issue of West Coast Avengers, apparently suffering from Laser-Guided Amnesia. This was supposed to lead to her becoming a member of the team, but due to Englehart finding out that he was being taken off the book and replaced with John Byrne, this had to be dropped. Instead, the story was abruptly resolved two issues later, with the West Coast Avengers learning that "Mantis" was actually a plant simulacrum created by the real deal.
Kurt Busiek's Avengers run had the team face a powerful villain named Bloodwraith. They were unable to best him, but Scarlet Witch managed to bind him to the ruins of Slorenia, making it impossible for him to leave the country's borders. It was mentioned a few issues later that S.H.I.E.L.D. was still monitoring Slorenia to make sure Bloodwraith didn't try anything funny, but Busiek ended up leaving the title before he could revisit that plot thread. The next time Slorenia appeared, absolutely no mention was made of Bloodwraith or where he'd disappeared to.
Geoff Johns' first Avengers arc introduced a new Scorpio. Though his plan was thwarted, the ending showed that he had escaped with the Zodiac key, and that he was actually part of a new incarnation of the Zodiac group. Zodiac's mysterious leader mentioned that the next time, the group would attack as one to ensure the Avengers could not stop them. Unfortunately, there never was a "next time," since Johns left the book before he could continue that subplot. The new Scorpio and Zodiac never appeared again.
Bendis hinted at a romance between Clint Barton and Maya Lopez in New Avengers, even having the two share a passionate kiss. This was completely dropped once Clint's ex-wife Mockingbird returned "from the dead" (it's complicated) at the close of Secret Invasion (2008).
The first arc of the Heroic Age relaunch of Avengers had Kang reveal that Tony Stark had thought up a hypothetical doomsday weapon called the dark matter accelerator, which possessed enough power to destroy New York City. At the end of the arc, Tony's counterpart from a Bad Future where Ultron had wiped out humanity gave the dark matter accelerator to his present day counterpart, telling him to stop Ultron's rise to power no matter what. Despite the accelerator clearly being set up as a Chekhov's Gun, it ultimately played no part in the eventual Age of Ultron storyline, where Hank Pym wound up being the one to engineer Ultron's demise, not Tony.
Spin-off Comic Books
Avengers: The Initiative: Earlier issues had dark hints that Slapstick — a jokey character with Looney Tunes-like powers — was slowly going insane, like when he beat Gauntlet into a coma, or kept a device with the brain patterns of the murderous KIA for some purpose. He actually tries several times to talk about this, almost confessing to nearly killing the Gauntlet to his teammates and mentioning at one point that he can't go back to his Secret Identity as Steve Harmon anymore — but every time he does, he either gets interrupted or dismissed as more of his wacky antics. It was an interesting subplot — that ends up going nowhere, thanks to the events of Secret Invasion (2008) and Dark Reign completely changing the plot of the whole series. We never find out what he does with the brain pattern device, either.
During a crossover between the Avengers Academy and Young Allies, it was hinted that Reptil and Spider-Girl harbored an attraction for one another, and were going to hook up. The problem is, Young Allies got cancelled after just six issues, so their relationship was nixed with a single Hand Wave line.
Near the end of Nick Spencer's Secret Avengers run, Daisy Johnson was fired from S.H.I.E.L.D. and replaced by Maria Hill. Johnson was later shown recruiting the Winter Soldier for a plot to bring down Hill's regime and expose her shady dealings to the public. The book quickly veered off course into a tie-in to Infinity involving one of the new Inhumans, and abandoned the Daisy subplot before being cancelled. Other titles later established that Daisy and Maria had resolved their differences.
During the "Planet X" storyline, The Wasp and Havok had a daughter together named Katie while in the alternate future created by Earth's destruction. Katie was eventually kidnapped by Kang the Conqueror, who used her as leverage to get Havok to play along with his plans. Just prior to AXIS, Immortus showed up and gave the couple hope by telling them they could get Katie back by conceiving a child at the right date and time, which he claimed would allow Katie to be reborn in the prime timeline. Both Wasp and Havok were written out of the series after AXIS, and the subplot was later abandoned entirely once Remender left the book after Secret Wars (2015). The romantic relationship between the two was also ended offscreen by subsequent writers, removing any remaining hope for Katie's rebirth.
Rick Remender's run ended with The Vision learning a horrible secret about Scarlet Witch and refusing to divulge it in order to spare her feelings, and the High Evolutionary claiming that his genocidal actions were meant to safeguard the world from some unspecified oncoming threat. Both plot points were dropped when the book was cancelled and relaunched with a new creative team due to the aforementioned Secret Wars reboot.
Films
The New Avengers at the end of Avengers: Age of Ultron were hyped up as a replacement roster because of Tony retiring, Hulk going missing and Thor leaving to look for the Infinity Stones. They proceeded to do one onscreen mission in Civil War which lead to the eponymous conflict and then quietly disappeared until Avengers: Infinity War 3 years later.
Thor's Achilles in His Tent storyline does get a resolution when he comes back claiming to have had "a vision" but what was meant to be a longer storyline ended up mainly a Missing Trailer Scene.
The Maximoff twins have a major subplot involving them seeking revenge on Tony Stark for killing their parents. There's no real resolution to this, as they only side with the Avengers because they realize Ultron is an even greater evil, not out of any real epiphany about Tony not being such a bad guy.
Thor's vision of Asgard was supposed to setup Thor: Ragnarok. Depictions of the Asgardians descending into decadence, demonic musicians, and Heimdall being blinded were all meant to foreshadow the events of the upcoming film. Once Taika Waititi was brought onboard as the film's director, Ragnarok underwent a complete Tone Shift and much of the imagery seen in Thor's vision went unused.
Western Animation
The Avengers: Earth's Mightiest Heroes: Due to being canceled after only two seasons, the show had a few of these. Notably, the heavily foreshadowed showdown with Surtur and the Enchantress never took place, and Maria Hill's repeated rants about a Superhuman Registration Act never panned out.
Surtur and his Asgard shenanigans are this thanks to the cancellation of the show.
Beta Ray Bill's story and his people are left hanging. As well as any Ship Tease with Sif.
Amora's still captured.
When Thor had a hammer forged for Beta Ray Bill, he owes one favor to the dwarf king, and it can be anything, and the king looked as if he was interested in collecting.
Maria Hill's efforts to start a Super Registration Act and leading the heroes toward Civil War didn't come up after episode 28.
Abigail Brand's implied alien ethnicity.
Korvac's plotline never went anywhere after his debut, though Peter Quill essentially says "Leave this kind of stuff to us, you really don't want to know what else is out there," to both Tony and the audience.
Kang's final episode never resolved his main arc. Although the The Avengers: Earth's Mightiest Heroes tie-in comic lends some closure to him and Ravonna. Unfortunately for them, it's a massiveDowner Ending since they are both trapped in Ancient Egypt with no way out, which should potentially erase their future time period completely.
The romance between Hawkeye and Widow was never even acknowledged after she was revealed as the Mole.
Avengers Assemble Season 2 ended with Captain America and Iron Man deciding that they needed to expand the team in order to face bigger threats, and the last shot showed them looking at images of a bunch of potential new recruits like Black Panther, Captain Marvel, Doctor Strange, and Moon Knight. The Season 3 premiere then opened up with none of these characters as part of the Avengers roster, with The Falcon offhandedly mentioning that the expansion plan had been canceled. Later episodes did however find ways to incorporate some of the new recruits, but not in the original plan: they were part of a government sponsored team, then later filled in for the original Avengers when they disappeared.
Despite Captain America's promise to redeem Bucky, now the Winter Soldier, the show seems to forget about this plot point between each of Bucky's appearances. Half-way into season four, this has yet to be resolved.
Aaron Reece, aka Molecule Kid from the same-titled Season 1 episode. At the end of the episode, he (and Fury) are convinced that he should join up with Fury's SHIELD. Academy Hero Program, the same one that Spider-Man is in. Despite him departing with Fury in the end, and despite a rather powerful and emotion-filled episode introducing the character, Aaron is nowhere to be seen or heard after the episode ends. Not even the 3rd and 4th seasons of Ultimate Spider-Man which expands the hero count and even shifts focus to the Academy's HQ sees any hide or hair of Aaron.
