After spending the past year-plus rooting for Gary Mitchell I have now abruptly changed my vote to Lord Garth. Cuuuuz...
1. Cumberbatch looks more like a Lord Garth (supercilious egomaniacal nutcase) than a Gary Mitchell (who I'd expect to be cast with some fresh faced young guy, an evil version of Pine's Kirk.)
2. We can't have a villain named Gary. Seriously.
3. There were clues for both Khan and Mitchell in the extended version of the trailer. Since it can't be both, that means Abrams is frakking with us. Lord Garth it is!
I bet in earlier drafts they wanted Khan though, and couldn't make the villain's motivation work as well, as a bad apple who just came from within Starfleet to begin with.
I wouldn't be surprised if that were the case. It's only when you start thinking about Khan that you realize he's hard to work into a movie from scratch. Too much explanation and setup needed, so you'd have to focus and simplify him so much, you'd end up with someone closer to Mitchell or Garth anyway.
Because why bother rebooting a universe if you're just going to tell the same stories?
Why bother recasting Kirk and Spock? They could have gone with an all new cast and time period. They decided to reboot the main characters, so why stop now? To most of the audience, everything is new. Even Khan would be, and he's the only TOS villain with any pop culture awareness.