Spider-Man's old flame Mary Jane Watson could make X-Men history. Mary Jane is best known as Peter Parker's lover (and in a timeline that's now been rewritten, his wife). She's better known in the rest of the Marvel Universe as a fashion model and actress, though recent comics have revealed Mary Jane has become a prominent mutant activist. Unfortunately this has led to Mary Jane becoming a target.
In the FCBD: Avengers/X-Men one-shot, the story "Let's Talk About Krakoa..." by Gerry Duggan and Matteo Lolli reveals the X-Men's greatest enemy, Moira MacTaggert, has marked Mary Jane for death. She knows Mary Jane has been sent a ticket for the second annual Hellfire Gala, and Moira has her own plans for the event. She's decided to steal Mary Jane's ticket in a horrific way, one she previously practiced with her own former lover Banshee; namely, by killing Mary Jane and stealing her skin. It's unclear whether Moira knows of Mary Jane's connection to Spider-Man, though. If she doesn't, she could find this particular impersonation will prove trickier than she expects, because Moira is unlikely to be able to fool Spider-Man's spider-sense.
Marvel has published a Hellfire Gala Official Guide online, and it hints Mary Jane will make X-Men history. It features art of Spider-Man's Hellfire Gala costume, and it implies the X-Men will modify their Resurrection Protocols to bring Mary Jane back: "Spider-Man didn't swing an invite to the inaugural Hellfire Gala, but this year, Peter Parker will find that his destiny is all too intertwined with that of the Krakoan mutants due to certain new developments in their Resurrection Protocols."
There's a sense in which this isn't really surprising. The X-Men comics wouldn't be allowed to kill such a high-profile supporting character in Spider-Man's world - and certainly not in such an abrupt manner. But it's difficult to figure out just how the X-Men's Resurrection Protocols will be modified to resurrect Mary Jane, simply because they involve a synergy of mutant powers and Cerebro technology. Professor X has recorded the mental patterns of every single mutant on the planet, and those records are uploaded into a clone body. He's never been known to have scanned Mary Jane, making this all rather odd. Clearly Marvel is keeping a lot of secrets about this upcoming event.
The first Hellfire Gala left the world shocked as mutants staked their claim to the planet Mars, but the second looks set to reveal their greatest secret: the fact that they have conquered death. Surprisingly, Mary Jane's story suggests the X-Men will agree to share the power of resurrection with humans, although they may well agree to do so only in specific circumstances. This at least means Mary Jane Watson, Spider-Man's greatest love, isn't going to be killed off for good in an X-Men comic.