So ever since The Avengers was announced in the credits of Iron Man 2, there's been a percentage of Marvel fans who have been crying out for them to release a crossover film bearing the Avengers vs X-Men branding. Here's the thing. Movie rights for Marvel characters are all over the place. But there is hope, young padawan.
Currently, 20th Century Fox owns the film rights to not only the X-Men, but also the Fantastic Four. There are many fan theories, and I've got one of my own, so if you don't like it, that's too bad.
The film opens with [The Avengers](movie:9040) chillin' in Avengers Tower, just hangin' out and being freaking cool. The scene cuts away to the Xavier Institute, where the X-Men are teaching classes, when all of a sudden, they pick up a distress call from Genosha, which is incredibly close to Latveria, where Doctor Doom has opened a portal, trying to establish a hold in the other parts of the multiverse, which throws the Avengers together with the X-Men, and, if they could get Sony to get kindly remove the stick from their collective ass, Spider-Man as well. The portal has unexpected consequences, and it creates a singular timestream where Doom reigns supreme over Earth, and where all the heroes have to work together to set the timeline right and separate their three worlds again.
The kicker is this, though. None of them remember who they were supposed to be, except for Spider-Man, Wolverine, and Thor, who was away in Asgard and thus outside of the effects of Doom's portal. They have to remind the other heroes who they were prior to Doom's takeover, and create a super team of sorts. They eventually set things right, and badabing, badaboom, Bob's your uncle, and the three parallel Earths are set back the way they were supposed to be.
To avoid the presence of too many heroes for one film, I would limit the cast to this:
Avengers: Iron Man, Thor, Falcon, Quicksilver
X-Men: Wolverine, Kitty Pryde, Quicksilver
Villains: Doctor Doom, Madame Masque (Black Widow in the real timeline), Trick Shot (Hawkeye)
This allows each team to have a good showing of competency. Plus, can you imagine the banter that would go on with two versions of Quicksilver thrown in? Especially with Evan Peters' version? That would be comedy gold. Plus, Wolverine and Iron Man are both pretty witty themselves, so throw them in with Spidey, and you have several good comedic relief moments available.
Like I said, this is just a fan idea, and you can take it or leave it.