Is Cumberbatchersnatch confirmed as the villain, or does he just have a major role in the movie? I know we've got pictures of him massaging Spock's neck, but...
My feeling about Gary Mitchell is that he can only work if he never gets exposed to that ridiculous energy forcefield in the first place. Then maybe he'll find some other reason to become a bad guy. It would cheapen WNMHGB though, as it would seem he wasn't really Kirk's friend at the academy if any number of circumstances could turn him against.
The thing is, once he gets exposed and develops psychic abilities (which I've always been sketchy on anyway, even though WNMHGB is my favorite pilot -- hell it's the only ST pilot I actually like) there really aren't that many ways the story can play out due to the conditions of the original episode. They're stranded on the galactic rim, out of gas, with only one service station nearby that may not even be there in this reboot because Zim went and moved it. This is why the remake version was already told in comic form (and as much as fans seem to have forgotten that, you can be sure they'll decide to remember once it does turn out to be Mitchell. Why just declare the film guilty when it can be twice as guilty?). You know, I think this would be worse than resurrecting Khan.
And then of course, Orci and Kurtzman still have yet to change their stance regarding the Ongoing comic series being canon with the films. They should at least have had a story outline, and known who their antagonist was, by the time the WNMHGB issue was being adapted.