So far we've had Magneto (Along with the brotherhood mutants) William Stryker (sort of) Dark Phoenix, and Sebastian Shaw.
What's next for the X-men? Sentinels would seem pretty obvious since Singer tried to get them in there since the first movie, and made a cameo of sorts in the third.
Another villain they could use would be Mr. Sinister. Perhaps set up a story involving him trying to control Havok, and use that to introduce his brother Cyclops (ignoring X-men Wolverine).
Apocalypse is one of the more out-there villains, but could work if he's modified a bit.
As for other villains, well X2 took a one-time villain from an 80's graphic novel and changed him enough to make a compelling villain (Although more in the Brian Cox version)...so anything's possible I guess.
What's next for the X-men? Sentinels would seem pretty obvious since Singer tried to get them in there since the first movie, and made a cameo of sorts in the third.
Another villain they could use would be Mr. Sinister. Perhaps set up a story involving him trying to control Havok, and use that to introduce his brother Cyclops (ignoring X-men Wolverine).
Apocalypse is one of the more out-there villains, but could work if he's modified a bit.
As for other villains, well X2 took a one-time villain from an 80's graphic novel and changed him enough to make a compelling villain (Although more in the Brian Cox version)...so anything's possible I guess.