3) Juggernaut
Had Matthew Vaughn directed X-Men: Days of Future Past, he would have used Juggernaut to help break Magneto out of prison. Bryan Singer later made the decision to replace him with Quicksilver (which definitely paid off), but hopefully whoever helms the next instalment will have some interest in this unstoppable bad guy.
Vinnie Jones’ take on Juggernaut in X-Men: The Last Stand may have put you off from ever wanting to see the character on the big screen again, but the right actor and redesigned look would work wonders. The comic book Cain Marko grew up with Charles Xavier as his step-brother, and that’s something which could very easily be added here, especially as Mystique played a role in Professor X’s childhood. Juggernaut even served as a member of the X-Men for a time, so there’s a lot they could to do with him both as a hero and villain.