2. They've Already Killed Patrick Stewart On Film
Sorry, folks. Charles Xavier's dead. Cyclops killed him. For now, until Marvel needs to sell more books and they bring him back. You can't backpedal on film like you can in comics. (Or maybe you can? I'm looking at you Spider-Man reboot). Xavier died talking Cyclops down from turning the entire world into an irreparable fustercluck. He didn't succeed. Similarly, Sir Patrick Stewart's Xavier died on celluloid in the deplorable 'X3: The Last Stand,' and despite his resurgence in 'X-Men: First Class,' played adeptly by James McAvoy. He's out of the picture on film for a while, kind of.
He's rumored to appear in 'X-Men: Days of Future Past,' due to a "time-travel element." But that doesn't count because those X-Men will not be fighting any Avengers, lets hope. Donner's X-Men films employ a hermetically sealed timeline free of Avengers and will continue to do so until someone else comes along to revamp the film mythos entirely; ala Christopher Nolan's treatment of Tim Burton's in the Dark Knight trilogy. Professor X served a meaningful, Obi-Wan-like role to Cyclops' (to quote 'Curb Your Enthusiasm') mopey dick Darth Vader. Cyclops kills Xavier and stoically repents, saying he'd want more than anything for their places to be switched. And I agree completely. In a perfect AvX, Cyclops would've been killed, allowing Marvel to re-invent a perennially, predictably stale character. But this is all a moot point. Xavier's dead. Sure, they could recast him, but why the hell would you go with anyone else besides Jean Luc Picard? I mean Patrick Stewart.