Proteus

This is a villain mentioned by Bryan Singer before the release of X-Men: Apocalypse. According to the director, he would make an interesting villain from a visual point of view. Given Proteus' abilities, I do agree with that.

Furthermore, the character was already teased in X-Men: Apocalypse. Upon meeting with Moira, Professor X notices that she has a son. And this should be all the introduction this character needs "“ Moira's mutant son, who develops a grudge against Professor X. And how could he break the most powerful telepath? By possessing Wolverine and making him slaughter innocents.

Yes, I know "“ Proteus has a weird weakness and cannot possess people who have metal in their bodies. But, this being an adaptation, things could change. And Proteus should be the first villain to be featured in the flick.