Proteus

Proteus is another prime example of a mutant who can possess any being he encounters. The son of Moira MacTaggert, he is an omega level mutant so powerful that his own body couldn’t sustain him. A psionic being, he must switch bodies in order to survive.
Besides Jean Grey/Dark Phoenix from the Last Stand, the use of Proteus in several X-Men movies would be the first time a mutant with reality-warping powers is presented. In the comics, he is kept in check after being sent into a coma by Professor X.
We can see that happening in the movies – but what if he merely fakes it, leaving his body at will in order to set his plans in motion? And what if, in the end, he decimates the X-Men, leaving only one to stand in his way?
This would be a perfect chance to reintroduce another fan-favorite character: Wolverine (if Old Man Logan will indeed be the final movie of Hugh Jackman). Why? Because Proteus has this ridiculous weakness, a sensitivity to metal…
