Since there's so much speculation about the villain, I may as well throw in my two cents.
Considering that they already did Gary Mitchell in the comics which, while perhaps not being canon per se, are at at least meant to be consistent with the movie timeline, I tend to discount Mitchell as the villain. I've seen suggestions that the movie will be a sequel to the comic, which seems like a really counter-intuitive method of storytelling. Especially if they want to make the movie accessible.
Yes, they did the prequel comics for the last movie, but those simply provided extra background material that the audience didn't really require. In this instance, there would appear to be too much backstory to quickly get the uninitiated up to speed. "Oh, look, it's Kirk's old friend who got superpowers and went crazy when we tried to fly outside the galaxy so we killed him and shot his body into space but now he's come back to life somehow and now he wants revenge. And he doesn't seem to have his god-like abilities anymore."
I mean, okay, it might be argued that much the same thing was done with Wrath of Khan- they essentially just gave us the TV Guide blurb version of Space Seed. Frankly, though WOK is naturally one of the better Trek films, I actually felt a little like I was missing out on something not having seen Space Seed beforehand.
My money is on Garth, but I could be wrong; I just think that would be cool. Plus, while there are plenty of good reasons against BC being Khan or Mitchell, as others have already mentioned previously in this thread, there doesn't appear to be any argument as to why he couldn't be Garth. I do think it's great that Abrams and co., in this age of rampant spoilers, have managed to keep so much under wraps so far.