IT IS ZEPHRAM COCHRANE
Escaped from the love embraces of the Companion he seeks his vengeance on Starfleet and earth for leaving him on that shit hole after ALL HE DID for mankind's future.
I realize your being facetious, but that's actually worth giving some serious thought to. Well, for a moment, anyway. Him somehow being the villain would certainly be more mind-blowing than it being Mitchell or even Khan.
Cochrane is definitely a wild card character, he could end up going in all kinds of directions. I think this would be a more fitting ending than what TOS gave him. And the Companion may have warped his mind a bit too, told him for centuries that he was alone because of earth and only she cared for him. He's insane and bitter.