Ronin from New Avengers was originally supposed to have been Daredevil in disguise, with early promotional material even strongly hinting at a connection between the two heroes. Brian Michael Bendis was forced to change his plans when he and Ed Brubaker agreed to end Bendis' Daredevil run with the title character in prison, which obviously made it impossible for Daredevil to be a member of the Avengers. Bendis ended up choosing Maya "Echo" Lopez as Ronin when he realized she had all of Daredevil's martial arts skills and his knowledge of the Japanese underworld.
The pitch for what eventually became Avengers Disassembled was originally titled Avengers: Chaos. The outline is mostly similar to the finished story, but with a few key differences. For one, The Vision was going to morph into Ultron and split into multiple copies during the attack on Avengers Mansion, rather than merely releasing a series of Ultron drones from within his body. Additionally, She-Hulk was supposed to escape the mansion and go on a rampage through New York in Avengers #501, only to be taken down by Thor, who would then tell his teammates that he had to leave due to disastrous events occurring in Asgard. This is in contrast to the published version, where Thor doesn't appear at all and She-Hulk is instead knocked out in an anticlimactic (and somewhat implausible) manner by Iron Man during issue #500. Lastly, Avengers #503 was supposed to end with the Avengers disbanding and Captain America subsequently approaching Iron Man with a proposal for a new team, leading directly into New Avengers #1. Instead, the team doesn't actually break up until the Avengers Finale one-shot (an issue not included in Bendis' initial proposal), and Cap doesn't get the idea to form a new Avengers line-up until New Avengers #3.
When Joss Whedon had to put Astonishing X-Men on hiatus to go direct Serenity, Marvel offered Bendis the chance to write a New Avengers/Astonishing X-Men crossover to fill the gap. As the project began to take shape, it eventually became clear that it was too big a story to just involve those two teams, and thus House of M was born.
Avengers #200, the issue featuring the infamous story where Carol Danvers was revealed to have been raped and later be Put on a Bus, was originally supposed to have a completely different conclusion. Carol's Mystical Pregnancy was supposed to have been the work of the Kree Supreme Intelligence, continuing a dropped plot thread from the cancelled Ms. Marvel series, wherein the Supreme Intelligence had expressed the desire to use Carol to create a new race of Kree / Human hybrids. The problem was, Marvel had already published an issue of What If? where the Supreme Intelligence used the dead body of Rick Jones to create a Kree / Human hybrid. Not wanting to essentially rehash a story that had already been told, the editor demanded that Carol's pregnancy arc be given a different resolution.
After writing a storyline where Quicksilver betrayed the Avengers and tried to kill them, Steve Englehart had wanted to keep the character as a villain and have him as a recurring menace. Editorial eventually pulled an Author's Saving Throw and revealed that Quicksilver had been brainwashed by Maximus, effectively nixing this idea.
During his Avengers run, Kurt Busiek had briefly considered giving Carol Danvers the new moniker of "Nemesis", in reference to her adopting a more vengeful attitude after her rape (Nemesis was a Greek goddess who was raped by Zeus). Busiek says he was never quite comfortable with the name, as he felt the explanation behind it would've been too convoluted, which is why he ended up renaming her Warbird instead.
Busiek had toyed with the idea of making Triathlon gay, but ended up leaving that idea on the cutting room floor after the unexpected backlash the character received for being a "Token".
Avengers: World in Chains was going to be a twelve issue series by Kurt Busiek and Carlos Pacheco, exploring a world where Captain America had never been unfrozen. The story was replaced by Avengers Forever, with an Easter Egg featuring some of the character designs.
After the original West Coast Avengers mini-series, the original plan was to use the regular Avengers book to feature BOTH teams in alternating stories (as seen in issue #250). But the team's popularity put the kibosh on the whole idea. Also the Shroud would have been part of the team.
According to Dennis Hopeless, Avengers Arena started out as a completely different series that was entirely focused on the Braddock Academy kids, but the higher-ups at Marvel felt that it had already been done. Supposedly, there was an arc where the kids fight each other, and that was the only part of the proposal that the executives found interesting, so Hopeless, seeking to salvage the project, expanded that arc into a whole mini-series, then threw in a bunch of other teen heroes who weren't being used in order to expand the cast.
Roy Thomas wanted to use the original Golden Age version of the Vision in his Avengers run, but Stan Lee told him no. Thomas ended up creating the better known Vision as an Expy of the original.
Princeless author Jeremy Whitley has said that at one point, he was slated to write a story starring America Chavez and Kate Bishop for the Secret Love one-shot from Secret Wars (2015). The story was shelved due to the characters being used in Siege, though a similar story starring America and the Marvel 1602 version of Kate was later published in the Secret Wars Too one-shot.
John Byrne wanted to have Hank Pym and Janet van Dyne get remarried while he was writing West Coast Avengers, but editorial wouldn't allow it.
Young Avengers: Hulkling was originally pitched as a girl who posed as a guy when fighting crime; Wiccan was going to struggle with the fact that his love interest was sometimes male. It's been speculated that creator Allen Heinberg thought this was as close as Marvel would let him get to putting an openly gay couple on the team. Eventually he had a change of heart and asked for permission to make Hulkling 100% male. Other speculations are that he changed his mind because "genderbender-gay" romance idea was being used in Runaways.
On the other hand, Brian Michael Bendis and Tom Brevoort's steadfast refusal to allow Heinberg to outright overturn Avengers Disassembled via bringing back Scott Lang as Ant-Man and redeeming Wanda is why Heinberg bailed upon the title after the first 12 issues. Story notes however, such as Heinberg's plans for a rookie villain version of the original Masters of Evil led by an android version of Egghead were ultimately written by other writers, and the Avengers: The Children's Crusade miniseries seems to have accomplished the goal of resurrecting Ant-Man and bringing Wanda back.
Duncan Rosenblatt, the main character of Firebreather, was originally conceived as part of a Young Avengers series. When plans fell through, his creator simply reused the concept at Image Comics. Had he been published by Marvel, Duncan's father would have been Fin Fang Foom.
More specifically, the Young Avengers-type book would have been called The Crew, with the cast consisting of teenage clones of Marvel heroes created by the A.I.M. organization.
As Mister Fantastic and the Invisible Woman had recently been written out of the Fantastic Four at the time, Walt Simonson had thought it would be interesting to have them join the Avengers. The editors changed their mind at the last second, so Simonson was told he'd have to get rid of them. The experience (plus the Executive Meddling he'd already experienced earlier in his run) pissed him off so much that he ended up resigning from the book.
Both series that Kieron Gillen wrote with Kid Loki had this:
Journey into Mystery was supposed to be about adult Loki - Gillen claims that, while he was aware Matt Fraction resurrected the character as a kid, he thought it to be temporary. If he went with the original plan, the story would be a much darker Spiritual Successor to The Elric Saga.
Young Avengers vol.2 was supposed to have an all original cast and several new members from the start, but Gillen realized the opening story would be too crowded and cut it to the bare minimum. It was also supposed to be a Spiritual Successor to X-Statix. Gillen wasn't allowed to use Patriot, because another writer already pitched a story involving the character, and when looking for another teenage Captain Geographic to fill the niche he thought of Miss America. Gillen also noted that with the addition of Loki and Miss America and their brash personalities, there would have been no room for Speed to stand out and with plot already revolving strongly around his brother it led to him being Put on a Bus. Gillen also once said that he almost didn't took the offer to write Young Avengers, fearing that Alan Heinberg's run would be too much of a Tough Act to Follow.
Way back in the 90s, Rob Liefeld and Jim Valentino pitched a Young Avengers series, but it was rejected for being too similar to the then-in-development New Warriors series. The series would have starred Namorita, Speedball, the teen version of Vance Astro (the character that eventually evolved into Marvel Boy and then Justice), Firestar, and Richard Rider, who would have gone by the name "Torpedo" after having lost his Nova abilities. Several new characters named Brahma, Photon, Cougar, and Combat were also planned to appear, and would later end up recycled for Liefeld's Youngblood (Image Comics) series.
The Ultimates: In interviews, Joshua Hale Fialkov had said that his first arc ("Ultimates Disassembled") would lead to a brand new team of Ultimates, with the book now following these new characters as they tried to fill the void left by the originals. Because of the low sales on the Ultimate line, the book was cancelled as soon as "Ultimates Disassembled" ended, and was instead relaunched as All-New Ultimates after the Cataclysm: The Ultimates' Last Stand crossover.
After his appearance in the first Secret Avengers story, the writers had plans to include Nova in more SA story lines. Unfortunately, Dan Abnett and Andy Lanning called dibs on having the character and killed him off during The Thanos Imperative.
Uncanny Avengers was originally supposed to have Brother Voodoo as part of the team, but Marvel refused to allow Rick Remender to resurrect the character. This is at least partially why the team ended up with such a white line-up, according to Remender.
After Ares got killed, Jonathan Hickman wanted to resurrect him on the pages of Secret Warriors, but Tom Breevort stopped him, saying it would be too early. Similarly, Breevort stopped several attempts at resurrecting The Wasp, knowing that Brian Michael Bendis, who killed the character, planned to bring her back in a story few years after her death.
Had A-Force continued, the next arc would've been a team-up with Black Widow, and would have seen America Chavez join A-Force as the team's replacement for She-Hulk. The following arc would have revealed Dazzler was briefly the queen of an alien planet at some point, and her former subjects would now beg her to return.
David Walker originally wanted the Native American member of Occupy Avengers to be Winona Wingfoot, the younger sister of Wyatt Wingfoot. However, his editors suggested that he instead use Red Wolf, whose book had recently been cancelled.
Avengers: Start Your N.G.E.N.S! was going to be a limited series about the Avengers teaming up with an Original Generation team associated with defense contractor Northrup Grumman. Said team was a four-person Gender-Equal Ensemble known as the Northrup Grumman Elite Nexus (consisting of Alyssa Woo, Hector Soto, Orville Norwood, Heidi Barber), and they would have used their expertise in aerospace technology and STEM education to save the day where the Avengers fall short. Issue #0 was published as a webcomic before being removed from Marvel's website. The rest of the series, which would have featured a Contest Winner Cameo, was cancelled due to protests over the nature of the comic.
An early teaser cover for Waid's first issue of the adjectiveless Avengers series (following his brief All-New, All-Different Avengers run) showed Doctor Doom as part of the team. Not only that, but Doom's Iron Man armor looked completely different than the one he actually wound up wearing in the Infamous Iron Man series, including clawed fingers, a green and gold color scheme, and no cloak. By the time the book actually launched, Doom was not in the starting roster, but the "Coming this year" teaser at the end of the first issue showed Doom telling the Avengers that they worked for him now, strongly implying that he would be joining the book at a later date. While Doom did guest-star in two issues, he never actually joined, as the editorial decision to do the Avengers: No Surrender storyline before relaunching the series under a new writer (Jason Aaron) ultimately kept Waid from executing many of his longterm story plans.
Speaking of the teaser from the first issue, it also hinted at a much more tempestuous crossover with the Champions, with both teams shown fiercely fighting one another. While there was definitely some tension between the two groups when they eventually met during the "Worlds Collide" crossover, they never actually wound up fighting each other. The final issue of the crossover did have a very brief altercation where Nova blasted Hercules and the Hulk punched the Falcon, but things never escalated from there.
