3. Magneto & Xavier Might Just Be BFFs Again
If it is to be the Age Of Apocalypse story-line (and perhaps that's just an over-excited presumption based on the use of the Days Of Future Past,) that would require a change in the dynamic between Magneto and Xavier (provided it is Xavier who is to die as the trigger for Magneto's change of ideology) to bring the two together as friends and allies once more. Obviously, the plot of Days Of Future Past will centre around Wolverine going back in time (through his consciousness, helped by Kitty Pryde's phasing powers, you have to assume) to stop Magneto and his team taking out their target, and inspiring the government to back Trask Industries' advancement of the Sentinel programme. If Magneto is going to die, and make Magneto change how he sees humans enough to completely change his ideology, he will genuinely have to care about his old friend again, and not see him as a necessary casualty in his greater goal. It might be somewhat presumptuous, but that intense feeling probably wouldn't happen with the two facing off against one another like they are at the end of Days Of Future Past. Magneto is resolute that everything is expendable in his quest to make mutants the ruling race, and Xavier is probably no more than an old distraction. So, something along the line has to happen for Xavier and Magneto's relationship to be repaired, unless we're going to see another time travelling element brought in where Xavier's killer goes back to the point in time that Xavier and Magneto were friends - but then, that would wipe out the importance of any of the events in Days Of Future Past, which would seem somewhat counter-productive.