X-Men First Class 2: Days of Future Past
Plot
With First Class taking place in the early 60's there's a lot of interesting potential candidates for the assassination target. Aside from the super obvious choice of John F. Kennedy (He after all did a good job of disguising that the mutants had any involvement in preventing World War III during the Cuban Missile Crisis, which no doubt will have pissed Magneto off) there's also his less famous brother Robert. There's also civil rights icons like Martin Luther King Jr. and Malcolm X. The obvious objection to using any of these figures is of course the question "When the good guys win, how do you handle the ramifications of a world where one of these great figures lived instead of died?" This would of course be a huge problem, but it's based on a very faulty assumption. The heroes don't actually have to win! You wanna talk about standing out from the crowd of superhero flicks? Nothing would stand out more than a movie where the heroes put forth all their effort and fail. Then again, there's no reason a First Class sequel have to pick up so soon after its predecessor. If the film makers wanted a more satisfying ending to their story there's a few real life examples of failed assassination attempts that could be used. The 70's offer up George Wallace, Gerald Ford and Jimmy Carter while 1981 has Ronald Reagan and Pope John Paul II. The attempts on Reagan and the Pope being so close in time frame (March and May of the same year) would create a chance to have the villain(s) of the piece attempt a back up plan when their first effort is thwarted.
Most of the characters that could be used in X4 could also be used for a First Class follow up. Technically all of them could be used via time travel tapping into the time frame taking place past the the third movie, but that runs the risk of getting a bit too convoluted. If the film were to take place in the 80's it would be after the 2009 prequel X-Men Origins: Wolverine and there would be an opportunity to bring in Wolverine and Sabertooth. Now before anyone objects with a "Wolverine couldn't be involved in a past plot with the X-Men if he doesn't remember them in the first movie!" take a moment to recall that in X2 Magneto chides Xavier for not having told Wolverine the truth. Well what truth could Xavier and Magneto be aware of involving Logan? Perhaps it could be something to do with his part in thwarting an assassination and having his mind wiped. This adventure could also offer up an opportunity to explain how Sabertooth goes from being such a cunning bastard in Origins to being dumber than a bag of hammers in X1. Whether Fox do Days of Future Past as a follow up to First Class or X3 (or even a separate movie altogether) the canvas is wide open for what kind of story can be painted for movie audiences.