I had to vote "other," though I really wanted to see Gary Mitchell in this one (followed closely by Garth). Both are great characters that would be enjoyable in a feature-length film. But what's available in the teaser and the 9 minute synopsis cuts against both.
Can't vote for Khan because I think that JJ is screwing with us on that point. The more the Khan clues appear, the more I doubt. In fact, I doubt so much now that if by some odd happenstance it IS Khan (or some Khan rehash), I'll sentence myself to shotgunning three seasons of iCarly in a 72 hour period.
Not Charlie X either. His powers were too similar to Gary Mitchell's... i.e. no BFG needed.
Harry Mudd... I can't even seriously consider there's enough there for a summer blockbuster film.
Trelane, no. He's quintessentially Q.
Gary Seven intrigues me, but he was an agent on a relatively peaceful mission and that doesn't fit the available information about Into Darkness.
Garth of Izar seems to fit best, but BC is out for revenge, he's doesn't seem to be the "dandy" narcissist that Garth most definitely is.
Honestly, over the last two days I keep coming back to Lazarus, or some variation of "The Alternative Factor" plot. In reading over the wikipedia synopsis, obsessed-Lazarus has similarities to what we know of BC's character. He's hell-bent on killing his alternative counterpart and he doesn't care that this confrontation would destroy both universes. It also fits in with JJ's fascination with alternative universes that FRINGE has explored. But for each similarity I find, there's plenty to de-bunk it.
Hence, I voted "other."