the way i see it happening, or how i would like it to be, in a 4th movie is bring back juggernaut, sabertooth, pyro (callisto is dead) and have mystique go to the x-mansion and be with them, assuming they get their powers back. rogue didnt' go anywhere so if she gets her powers back, we'll see, magneto on the other hand, what more can we expect to get from him that he already hasnt done?
i can see the mansion getting along with mystique, she never deliberately hurt anybody other than wolverine, but never magneto, that would make that last fight pointless, so either he dies of old age or we never see him again, we'll just have to wait it out i guess n see if a 4 is even made