The most notorious "what might have been" in the history of the X-Men is that Chris Claremont originally planned to end The Dark Phoenix Saga with Jean being completely depowered, rather than killed. However, Jim Shooter declared that Jean had to die, as during her rampage as Dark Phoenix she had been seen to destroy an inhabited solar system, killing millions of harmless aliens. This was especially annoying to Claremont, as the mass murder had been something John Byrne threw in when he drew the passage in question, and in the original script there had been no hint that the solar system had sentient inhabitants.
A lesser known detail is that death wasn't the original fate Shooter proposed for Jean. He originally wanted to have Jean be sent to an interstellar Shi'ar prison, effectively Putting Her On A Bus as well as having her pay for her crimes. Claremont and John Byrne felt it would be extremely Out of Character for the X-Men to just allow one of their comrades to languish in a prison for the rest of her days, so they came up with the idea to simply kill her instead.
X-Men #12 was a "tryout" book for legendary artist Alex Toth, as Stan Lee had wanted to see if he could get Toth to join Marvel. Toth ended up not liking the "Marvel Method," and thus declined to do more work for the series. Toth's original design for the Juggernaut was also far different and much more over the top, complete with spiked nipples for some reason. He was asked to tone down and simplify the costume, and that's how we ended up with the iconic Juggy we all know and love.
Early on, Stan Lee had considered revealing that Magneto was actually Charles Xavier's brother. He presumably recycled that premise for the Juggernaut.
Banshee was originally going to be a woman, which explains why the character was named after a female spirit from Irish mythology. However, Stan Lee vetoed the idea because he thought it'd look bad for a group of young men to beat up a female villain. The original concept would be reused for Banshee's daughter Siryn.
Len Wein originally wanted to call Multiple Man "Zerox" as a play on the Xerox line of copying machines. Roy Thomas immediately shot down the idea and told him Whos gonna pay off the huge lawsuit, you?
In the early 70s, there were plans for a split-book featuring Iceman and Doctor Strange, but the book never came to fruition.
X-Factor #78 has the Mutant Liberation Front attacking a scientist who has developed a way to identify the X-Gene in unborn fetuses. The story was originally going to have the scientist performing abortions on said mutant fetuses, with Wolfsbane's religious pro-life stance factoring into the story. Marvel dropped that aspect due to the inevitable backlash that would have ensued.
Giant Sized X-Men #1 gives the impression that right before the original X-Men series got canceled, both Havok and Polaris were active members of the X-Men. However if you go back and read those issues (along with the X-Men's guest appearances while the comic was in reprints) you'll see that they never actually went out with the rest of the X-Men on missions. Which is unfortunate, since this is one of the few things that could have made the Thomas-Adams era even better.
Giant Sized X-Men was also going to continue as a quarterly title, but the entire Giant Sized line was eventually cancelled due to poor sales. The contents of X-Men #94 and #95 were originally supposed to make up Giant Sized X-Men #2.
FOOM magazine ran a "Create-a-Villain" contest in 1973, and the winning entry was a character called Humus Sapiens. It was announced that Roy Thomas would be incorporating Humus as a charter member of the new, international ReTool of the X-Men (which would eventually become the All-New, All-Different X-Men), but when Thomas left the book and was replaced by Len Wein, the character was dropped. Humus Sapiens finally made a proper debut decades later in an issue of Thunderbolts.
While everyone knows about Thunderbird's death, the original plan for the All-New, All-Different X-Men was to have three members "flunk" the test to become official members of the team. Thunderbird was going to be one of them, while Sunfire and Banshee were going to be the other two. However, this idea was abandoned, which is why Sunfire randomly quit the team in the very next issue.
In the late 70s, there were tentative plans for a Spin-Off called The Furies, which would have seen Storm as the leader of a team of female superheroes. The group's other members would have been Tigra, Namorita, Clea, Dragonfly of the Ani-Men (explaining a dropped plot thread from X-Men #104 involving Dragonfly's escape from Muir Island), and a new alien heroine named Moon Fang. Dave Cockrum was supposed to launch the series, but never got around to finalizing the first plot.
Long before Wolverine became the poster-boy for the X-Men books, he just barely escaped being killed off soon after joining the team. An editor was annoyed that Wolvie was so similar to Thunderbird (the other rebellious bruiser who was introduced in revamped lineup), and demanded that one of them be killed off in the Count Nefaria mission. In the end, Claremont decided to kill off Thunderbird, as Wolvie had a more distinctive powerset and appearance. Three decades later, Thunderbird is barely remembered and Wolverine is one of the most iconic superheroes ever created. Go figure. Speaking of Thunderbird, Scott Lobdell and Aaron Lopresti did a two-issue Thunderbird mini-series to celebrate the 25th anniversary of the All-New, All-Different X-Men. However, this was right around the time Chris Claremont had returned to the franchise and introduced a brand new Thunderbird, and this, coupled with plans for a different 25th anniversary tribute, led to the mini-series being shelved indefinitely.
Claremont had wanted Nightcrawler to be Jewish, but both John Byrne and Roger Stern argued that it was really unlikely that there were many Jews of Kurt's age group in Germany at the time. They also felt that him being raised by a Roma family was already an interesting enough background without having to bring Judaism into it. So instead of a Jew, Kurt became a devout Catholic.
Some people, including readers, believed we were making too broad a statement. We had never openly declared that Northstar was gay. Now we had the only fairly popular Marvel character generally acknowledged as being gay and he was dying of AIDS. You shouldnt equate one with the other.
James Hudnall's run famously had an Aborted Arc dealing with Zeitgeist, a serial killer obsessed with murdering superheroes. Had the story continued, Zeitgeist would have infiltrated Alpha Flight HQ and killed Goblyn before being taken down. The character was ultimately picked up in Captain America instead, where he became involved with a German team of superheroes, before being retconned into the character Everyman.
In the 90s, John Byrne was approached to do a series called North and South, which would have starred Northstar and Ironclad. The book would have been revolutionary for the time, as it would have treated Northstar's sexuality just like that of any straight character, which was a fairly radical notion back then. Higher-ups ultimately cancelled the project and cited budgetary issues, but Byrne has said he believes the fact that Northstar is gay also had something to do with it.
Mystique was going to be revealed as not Nightcrawler's mother, but his father, having impregnated Destiny while in the form of a man. Executive Meddling prevented this one, as it was expected to be controversial. Fans however seem to like the idea, and occasionally a writer will suggest retconning it into the truth (particularly since the reveal years later of Nightcrawler's actual father, who is literally the Marvel equivalent of Satannote as if Marvel really needed yet another Satan figure, given that there were already three major and countless minor Satan expies, has pretty much caused massive Fanon Discontinuity and is among one of the main reasons Chuck Austen has become a pariah among comics fans).
Mister Sinister was originally supposed to be the psychic projection of a kid who started stalking Scott Summers when they were both in an orphanage together, essentially making him a twisted pastiche of the Golden Age Captain Marvel. The fact that he was the invention of a child was the explanation for his (frankly) ridiculous name, and his entire look.
Claremont had considered revealing that most of Sabretooth's appearances prior to 1988 were actually clones created by Mr. Sinister, explaining the embarrassing defeats the character had suffered before he was revamped as a serious threat. He'd also wanted to reveal that Sabretooth was actually Wolverine's father, explaining why he was obsessed with proving Logan wasn't better than him.
Related to that, John Byrne has said there were plans for a story where Mariko would have been brutalized by Sabretooth, resulting in her being rendered brain dead. Heartbroken, Wolverine would have cut her life support to let her die with dignity, leading to a final confrontation with Sabretooth. The fight was to have ended with Sabretooth being Killed Off for Real and Logan learning that Creed was his father all along. What makes this interesting is that this was all planned to occur in 1981, five years before Sabretooth and Wolverine met on-panel for the first time in the Mutant Massacre story arc.
Gambit was supposed to be Claremont's Take That! response on New Teen Titans villain Terra, as far as being a spy Mr. Sinister hired to infiltrate the X-Men, right down to seducing Storm to get the X-Men leader's confidence. Also, back when Claremont was still going with the idea that Mr. Sinister was merely the projection of a mutant child, the plan was for Gambit to be another "creation" of that same boy. Just as Sinister was conceived as a little boy's idea of a scary villain, Gambit was supposed to have been a little boy's idea of a badass hero. Eventually, Gambit's love for Rogue would've caused him to develop a conscience and turn against his creator.
He also planned on using the Fury, created by Alans Moore and Davis in Captain Britain. The Fury would have merged with Nimrod, and ultimately been responsible for the Mutant Massacre, as well as teaming up with James "Mad Jim" Jaspers, another Moore/Davis creation, to destroy the X-Men. A dispute between Marvel and Moore over compensation note due to the differences in copyright law between the US and Britain. led to editorial nixing this version, and instead we got Mr. Sinister and the Marauders, and the Adversary, respectively.
Since Marvel UK existed in a murky position vis-a-vis the main series in America, Claremont's plan seems at various stages of the development of this story seemed to have been an adaptation of Moore and Davis's "Jaspers' Warp" storyline. Claremont envisioned this as a potential company-wide crossover which could function as Marvel's answer to Crisis on Infinite Earths, with the effects of Jaspers's reality warping powers appearing in other Marvel comics published that month. Although the plans fell through, Jim Jaspers did receive a cameo in Uncanny X-Men #200 as the chief prosecutor at Magneto's trial before the United Nations to set him up as a future participant in this story.
John Byrne intended for Pyro to be gay, with the flamboyant civilian attire and effeminate mannerisms he displayed in his first appearance being a major hint. For whatever reason, Claremont decided against this and made him Australian instead.
There was also the notion of a lengthy plotline where Wolverine is turned into the brainwashed minion of "The Hand"note This actually happened in a Mark Millar storyline decades later, only the Hand teamed up with HYDRA at the time, kidnapping Jean Grey to become his "Queen of the Night", leading to Forge and Banshee having to rescue Jean.
This "Dark Wolverine" story was later repitched as the opening storyline for the 1991 X-Men series: rather than fighting Magneto and the Acolytes, the X-Men would fight the Reavers, of which Lady Deathstrike would kill Wolverine via ripping out his heart. But the Hand (revealed to be in league with the Shadow King) would obtain Wolverine's corpse and recreate his heart/resurrect him as an agent and have him reappear in Uncanny X-Men #294, as part of the rematch between the X-Men and the Shadow King and his army of minions, as the Shadow King (via Gateway) seeks to gain control over everyone's dreams.
The infamous Race Lift for Psylocke was intended to be temporary, and was only supposed to last until the end of the Acts of VengeanceCrisis Crossover. The change proved so popular that Claremont decided to keep Psylocke Asian, and this became the character's default appearance in the movies, games, and TV shows until 2018.
Related to that, Psylocke's death in X-Treme X-Men (see below) was only supposed to be temporary. The plan was to kill off Psylocke and then resurrect her in a form closer to her "classic" incarnation (meaning no more Crimson Dawn powers or facial tattoos), possibly restoring her original Caucasian appearance as well. Marvel vetoed this since the story was planned during a period when the company was trying to avoid bringing back dead characters, which led to Psylocke's return being pushed back until long after X-Treme had been cancelled.
Claremont and Byrne almost did a What If...? issue about Magneto forming the X-Men after Charles is killed by Lucifer. The X-Men would have essentially been an expanded version of the Brotherhood, including Cyclops, Quicksilver, Iceman, Beast (who would have had mechanical Wolverine Claws), Archangel (who would have wielded a flaming sword), Scarlet Witch and Jean Grey (who would have been called Psyke instead of Marvel Girl). The issue would have seen the mutants defeating the Fantastic Four and Doctor Doom (with Magneto taking advantage of Doom's metal armor to kill him), only to be forced to save the world when The Coming of Galactus occurred at the end of the story.
Kitty Pryde was originally created as part of a group of young mutants, as Claremont and Byrne had wanted to bring back the school aspect of the Xavier Institute. Her teammates were to have been a young Reality Warper named Willie Evans (who had previously appeared in Fantastic Four) and a monstrous hillbilly teen named Caliban (no relation to the Morlock who would later be introduced with that name), who would've had the power to project his life force into inanimate objects. The plans for the team were squelched by Jim Shooter, but ended up being recycled to some degree for the later New Mutants series.
Claremont's return to the franchise in 2000 was supposed to feature a new mutant named Sketch joining the X-Men. Sketch actually did show up in a story, but Claremont ended up being forced off the books before he could make her an official member of the team.
Beast was originally going to be part of the X-Treme X-Men, and Claremont plotted out three years of stories featuring the character. This was scrapped when Grant Morrison pinched Beast for the New X-Men.
Though the idea was picked up in the X-Men Forever series, Chris Claremont had a started out with the idea of "mutant burnout", in which a mutant's power would effectively burn themselves out, leading to their death. This was seen in X-Men (1991), where Magneto was growing weaker and was warned that he was most likely dying from it.
Naturally Claremont was not a big fan of Jean coming back in X-Factor when he found out about and quickly pitched an alternate idea to Jim Shooter: that Jean's spot would be taken by her sister Sara Grey. Claremont had written a story the year before where Sara was kidnapped by Attuma and altered with an Atlanteon physiology before Jean used her Phoenix powers to alter her back. Claremont proposed that Jean doing this activated Sara's latent mutant power which was the ability to activate the powers of mutants close to her which would also work with X-Factor's modus operandi (as the X-Terminators) of finding mutants. Shooter liked the idea but felt the plans with Jean were too far along to change. Sara would be unceremoniously killed off panel around the time of the Phalanx Saga so Claremont subsequently took the idea and grated it onto an obscure character from his original run named Tessa thus leading to the creation of Sage.
Claremont and Dave Sim plotted out a crossover between the X-Men and Cerebus, but Sim lost interest after seeing Jim Shooter's lack of enthusiasm for the project.
Dave Cockrum designed an amphibious mutant heroine named Silkie, whom he'd intended to have join the X-Men in issue #150. However, he wanted to retain ownership of the character, something Marvel would not allow. He ended up recycling the character for his creator-owned series The Futurians.
Chris Claremont's Phoenix mini-series pencilled by Rick Leonardi, further exploring the future relationship of Rachel Summers and Franklin Richards, later becoming the story "Days of Future Present".
Chris Claremont's Excalibur Special Edition pencilled by Rick Leonardi, featuring Shadowcat and Phoenix, later becoming the mini-series "X-Men: True Friends".
Uncanny X-Men #209 ends with Spiral abducting Rachel Summers. Rachel returns in Excalibur: The Sword is Drawn, and references her time in the Mojoverse throughout the series. The story was planned to be shown in a limited series or graphic novel.
The division of the franchise into two books in 1991 might also have gone differently. A piece of original pin-up art by Jim Lee shows Wolverine, Beast, Forge, Strong Guy, Jubilee (Marvel Comics), Psylocke, Storm (in an unused costume) and Rogue, with Magneto looming in the background. Another shows Xavier, standing up, with Jean in an updated Phoenix costume, Storm (in the familiar '90s costume, suggesting this one was drawn later), Wolverine, Colossus, Gambit and Beast. There is a similar piece by Whilce Portacio that includes Cyclops, Jean, Archangel, Iceman, Gambit and Colossus with Xavier behind them. It's also been said that at some point Xavier would have been killed and Gateway, of all people, would have mentored some of the mutants. Also, let's not forget Jim Lee's "Things to Come" illustration with a creepy Skrull woman and Selene alongside Matsu'o, Omega Red, Longshot and Dazzler. Selene was there because she was the leader of the Upstarts, Lee's replacement for the Hellfire Club. Unfortunately, Selene was put on a bus when Lee left Marvel, as far as Bob Harras and Fabian Niceza deciding to use Gamemaster instead as the Big Bad.
The X-Men villain Apocalypse had been suggested as both the mastermind behind the Weapon X program (a plan fitting an immortal mutant with advanced technology and a penchant for playing god), as well as the third Summers brother who was sent back in time (explaining his and Mr. Sinister's obsessions with the Summers bloodline). Instead, Weapon X became part of Weapon Plus (with Captain America and Nuke being part of said program, as Weapons 0, 01, and 07) and the third Summers brother was sent to his death by Professor X, who then wiped Cyclops' memory when he got upset about it, but that brother came back to life with super-charged powers and conquered a galactic empire.
Speaking of Apocalypse, he was almost never invented at all. The first issues of X-Factor featured a mysterious master of the group known as the Alliance of Evil; writer Bob Layton was fired after five issues, and was replaced by Louise Simonson, who, along with editor Bob Harras, decided to create a new villain to be that mysterious master, and gave us one of the most dangerous X-Foes. Layton's original choice for the shadowy figure, however, was...uh...the Owl.
And speaking of X-Factor, Dazzler was originally supposed to have been the fifth member of the team. The final issue of her solo series even ended with Beast suggesting that she join the group. She was cut from the roster when John Byrne decided to bring back Jean Grey instead.
Additionally, Chris Claremont had suggested Jean's sister Sara Grey as the fifth member of X-Factor, pointing out that a scene where the Phoenix had altered Sara's genes in Bizarre Adventures #27 could be used to retroactively make her a mutant. Jim Shooter apparently liked the idea, but was already committed to bringing back Jean by that point.
Whilce Portacio and Karl Altstaetter originally wanted Bishop to be Filipino, but before they could lock down his ethnicity, Bob Harrass suggested that he be black, since the X-Men have historically had a large number of black fans.
Joe Kelly had wanted to have Jean become the Phoenix for real (since the original "Dark Phoenix Saga" had been Retconned to establish that it had never really been Jean), which would have caused tension with the X-Men who remembered what happened the last time Dark Phoenix showed up. The plan was to have Jean fight and struggle with her Phoenix persona, but ultimately win out and gain control, thus granting the readers the happy ending they were denied in the original tale.
Neal Adams was the original artist for the project, and even drew up the first six pages (including the aforementioned Magneto death scene) before being informed that Jim Shooter couldn't come up with a contract for him. He ended up being replaced by Brent Anderson.
Angel also appeared as part of the team in the pages Neal Adams drew.
As was mentioned, the book wasn't supposed to be canon, and in Chris Claremont's own words, William Stryker was never supposed to appear again. The villain's major role in X2: X-Men United motivated Marvel to officially declare God Loves, Man Kills canon and bring Stryker back to the franchise. He's since become a popular character despite his small number of appearances.
Adding to that, the scene in "Assault On Weapon Plus" where Logan consoles a drunken Cyclops was originally going to be Logan consoling a drunken Gambit after Rogue's death.
The original cast of the book was going to be different as well. In addition to Rogue, Morrison had wanted to use Storm, Colossus, and Moira McTaggart as part of the team, but had to change his plans when editorial informed him that Rogue and Storm were being used by Claremont, while both Colossus and Moira had recently been killed off. He ended up choosing Emma Frost and Beast to replace Colossus and Moira as the team's bruiser and scientist, respectively.
The U-Men were originally going to be called the Black Kross and Xorn was going to be called Experiment X.
Dazzlerwas originally modeled after Grace Jones, but Filmworks representatives wanted Bo Derek to play the role in the (failed) live-action adaptation, so she was redesigned to be a blonde white woman. Casablanca Records and Filmworks would then wind up backing out of the record and movie deal over financial concerns.
M in Generation X was intended by Scott Lobdell to actually be the twins Nicole and Claudette, with there being no real "Monet St. Croix" in sight (as it was only an alias they'd use in their merged form). Penance was also meant to be a Bosnian refugee by the name of Yvette. When Lobdell left and Larry Hama took over the title, he went with the idea of the twins merging but also opted to have there be a real Monet, who was trapped in the form of Penance. He explained away the "Yvette" references to Penance by stating that it was simply one part of Monet's overly long name note Monet Yvette Clarisse Maria Therese St. Croix. A rundown of the situation can be read here.
Also from Generation X, Jay Faerber's original pitch is vastly different from what eventually wound up being published. Examples include a new member of the team named Reach (an Icelandic pretty boy with portal-related powers), a new version of the group Salem's Seven who were a coven of witches who held sway over Monet, a vastly different role for a group called the Rising Sons (a Boston street gang and recurring foes in the pitch, muscle for a Madripoor crimelord who only appeared in one arc in the actual series) and most shockingly writing out both Banshee and Skin. Banshee would've left to spend time with a sick Moira on Muir Island while Skin would've gone with him in order to dodge cops looking into him for a drive-by shooting from his gang member days.
New Mutants (vol. 2) #8 and #9 had to be completely rewritten and redrawn at the last second over the controversial subject matter. The original story had Anole's parents get offended by the sight of Karma kissing her girlfriend during Parent's Day at the Xavier Institute, causing to Anole to come out as gay. After his parents reacted poorly, Anole would've gone to Hellion and Elixir for support, only to be shunned by them. Feeling alone and friendless, Anole was supposed to kill himself, cementing Hellion as an irredeemable Jerkass and causing Elixir to undergo some serious Character Development out of guilt. Bill Jemas was very antsy about the story, and it was ultimately pulped, leading to Nunzio DeFilippis and Christina Weir having to hastily write a new one in its place.
DeFilippis and Weir were also originally supposed to stay on the book after House of M and eventually shared some of their plans. This would've included a plot centered around the students going to Nova Roma to completely put to bed the entire issue of Magma's retconned backstory and Elixir eventually pulling a FaceHeel Turn due to his grief over the events of M-Day and subsequent attempts at trying to repower mutants. They'd also had ideas prior to House of M that they never got a chance to show, including normally background characters DJ and Preview going on a date.
Mark Waid's original suggestion for Onslaught was that he be the personification of Professor Xavier's long-suppressed dark side. He's since said that he feels the origin they went with instead was needlessly confusing.
The original The X-Men Vs. The Avengers mini-series (not to be confused with Avengers vs. X-Men) was supposed to end with Magneto (who at the time was an Anti-Hero and acting as the leader of the New Mutants) falling from grace and returning to villainy. Editorial changed their minds at the last second, resulting in the ending being hastily rewritten and a new creative team taking over for the final issue.
Gambit's concept art shows outfits with varying placement of belts, boots and pants, Gambit and a younger Storm working together as thieves, and a simpler uniform design by Igor Kordey for X-Treme X-Men.
The much hated Joseph was intended to actually be a de-aged Magneto, as was heavily implied in the comics. However, when it was decided that Magneto would be brought back as a villain, Scott Lobdell considered revealing that Joseph was actually a resurrected and amnesiac Proteus. In the end, he simply revealed that Joseph was a young clone of Magneto.
The story was originally going to be about Magneto tilting the Earth on its axis, sending the world into an ice age. Concept art by Leinel Yu shows the X-Men wearing winter gear in Egypt for this event.
The original concept for "Magneto War" was also bigger and far darker. The story would've seen Magneto trapping the X-Men in a mutant concentration camp as a warning of what was to come if mutantkind didn't stand up and fight back against humanity. The X-Men would've been divided over the issue, with some of the heroes siding with Magneto against their former teammates. Perhaps most shockingly, Storm was supposed to have been the big death instead of Joseph, as she would've pulled a Heroic Sacrifice to fix the planet after Magneto had tilted it on its axis.
Brian Michael Bendis was offered the job of writing Ultimate X-Men from issue #1. While he eventually declined the job, he did sketch out a script for issue #1. If you want to read it, you can find it at the end of Ultimate X-Men Ultimate Collection book 1.
Artist Declan Shelvey had plans for a Banshee-led X-Men series that had interest from the higher-ups, but ultimately went nowhere. It would have had Sean Cassidy, mysteriously and suspiciously healed from the Death Seed put into him during Uncanny Avengers, deciding to retire in Cassidy Keep and stay away from everyone else. However, his daughter Teresa would approach him looking for help and ultimately return to action, making his home a new safe haven for mutants. Sean's team would comprise of him, his daughter (who would also retain the Banshee name), Marrow, Chamber, Artie and Leech.
Rick Remender pitched a title called Extraordinary X-Men, which would have seen the X-Men struggling to rebuild the team after the school was destroyed by The Inhumans. Fantomex, Jean and Professor X (now known as Professor X-orn) all would've had major roles in the series, with a returned Mastermind serving as the Big Bad. The plans were axed when Remender left the company due to his dissatisfaction with the editorially-mandated Inhumans plot.
Relatedly, in 2019, Marvel approached Remender about possibly revamping the X-Men line, which he declined. The job instead went to Jonathan Hickman, who revamped the line with House of X/Powers of X.
Scott Lobdell originally intended to continue writing the X-Men after Operation: Zero Tolerance. O:ZT's original ending would've involved Magneto showing up and declaring war on humanity for allowing the events of the story to happen. The X-Men would've been the same group from that story: Iceman, Marrow, Cecilia Reyes and Sabra who had no access to the mansion, Blackbird or any of fancy Shi'ar tech as they're caught in the middle of a war between Magneto and Bastion. However editorial prevented Lobdell from making Sabra a member of the team and then subsequently wanted him to use something closer to the classic ANAD team from the '70s. Lobdell subsequently quit in protest though was eventually allowed to tell a heavily reworked and shortened take on his plans in the Eve of Destruction storyline just before Grant Morrison and Joe Casey took over the main books.
Chris Claremont planned to have Kitty being bisexual with the intention of Rachel being a possible love interest for her but was shot down by execs. The character would finally be revealed as bi in Marauders issue 12 in 2020.