Oh, man, forgive my nerdy transgressions, but I love trying to imagine where the next film will lead us.
Might they try again with Cyclops? He's a big part of what the X-Men stand for. In the comics, he's one of the few that understands Xavier's ideals- that should have come through better. And, as many say, we never saw him die. Who's to say he doesn't come back with some sort of mental rewiring courtesy of Mr. Sinister. After all, I'm sure other mutants in high places understand that the (temporarily) rudderless Xavier's School is a major chess piece in the struggle for mutant supremacy.
I see the opportunity to then introduce Gambit as one of Sinister's Marauders, who will try to enforce their willpower on the school before Sinister unleashes a modified version of the Legacy Virus designed to actually infect and kill all nonmutants. I can also see Apocalypse pulling the strings here- if they can avoid any and all story leaks and have him show up in the film's second half, well, that would be mind-blowing.
New teachers at Xavier's School would be needed. Storm needs some sort of arc that makes her realize that the regal African Queen may be wasting her time with the team- perhaps love interest Forge leads to her retirement? And how about new instructor Sean Cassidy, aka Banshee? Perhaps he can rejoin his daughter, Siryn, and bring along his love interest Emma Frost (after all, SOMEONE needs to use Cerebro).
I guess you'd HAVE to bring back Professor X. Maybe have him nurse himself back to health slowly while consummating a relationship with Moira MacTaggart. Question the amount of time dedicated to getting him back to his best health instead of letting the X-Men know he's alive. Some sort of twist, maybe then killing off Moira, can give Professor X something to do.
But what to do with Magneto? And do we need Wolverine?