3. House of M (Avengers & X-Men)
The House of M comic book arc would see The Avengers and the X-Men teaming together, though given that it follows the events of comics Planet X and Avengers Disassembled, it would also require some context that might be tricky to speedily convey in a movie. Essentially, Scarlet Witch goes mad and tries to change the fabric of our reality in order to conjure her lost children back, resulting in Peter Parker becoming a celebrity athlete, Cyclops marrying Emma Frost, Doctor Strange making a living as a psychologist, while Steve Rogers is merely an old war vet. None of them remember their former lives except Wolverine, who strives to fix the damage Scarlet Witch has caused, while trying to prevent Magneto from become a glorified dictator of the mutants, who now rule supreme over humanity. Though the upcoming Days of Future Past adaptation likely covers too similar ground for House of M to ever get an adaptation, it's still something we can hope for, and given that Scarlet Witch will be showing up in The Avengers 2, it's not
that unrealistic now, is it?