4) Juggernaut
Arguably one of the greatest X-Men villains of all time, the Juggernaut was handled in a predictably appalling fashion when he showed up being played by Vinnie Jones in X-Men: The Last Stand (“I’m the Juggernaut, bitch!”).
A reasonably big guy in a plastic looking padded costume is not the best way to handle this villain, and so it would be much better if the next iteration were to be enhanced by CGI. As long as they got the design and look of the character right, of course.
An interesting fact about Cain Marko is that he actually became a member of the X-Men for a time, but ended up leaving when it became apparent that his teammates would never view him as a hero. That could be worth exploring should Fox decide not to go down the route of bringing him back as a villain regardless of where he shows up (hopefully in a movie where the fact that he’s Professor X’s step-brother can be touched on in some way).