I have no idea whether or not there's going to be another entry in the X-Men series, but if there is going to be, here are ideas for villains previously unused in the first three films:
Apocalypse: The Darkseid of the Marvel Universe; also definitely one of the best villains in regard to putting the world in danger
Omega Red: Huge, strong, has adamantium whips that come out of his gloves, evil, and Russian - I don't know why they haven't used him, actually
Sauron: scientist that accidentally turns himself into an evil pterodactyl with a human brain; I'm guessing they might work in Ka-Zar and the Savage Land if they use Sauron
Then again, that's just my opinion.