2. Doctor Doom
Marvel ComicsWhen you think of X-Men villains, you immediately think of mutants (or, at the very least, mutates like the aforementioned Mister Sinister). However, with Fox's Fantastic Four reboot apparently being set in the same universe as the X-Men movies, why not have the X-Men facing off against Doctor Doom? Granted, Doom is set to be the villain in the Fantastic Four reboot movie itself, but if he proves popular there's no reason he can't appear again (look at Magneto and Loki for examples of this happening). Doom has a history of going up against a number of prominent X-Men movie characters in the comics - Magneto and Wolverine immediately spring to mind - and, as an ongoing antagonist in Fox's cinematic universe, it would be great to see him pitting his wits against mutants. Doom wears a powerful armour composed of advanced technology which grants him super-strength, incredible durability and a plethora of esoteric abilities. He is also adept in the use of magic - something that wasn't explored in his previous movie portrayal (Julian McMahon, hang your head in shame) and something that hasn't been remotely touched upon in the X-Men movies. He would be a very, very tough opponent for the live action mutants.