I definitely prefer that to Khan which is another popular rumour. I didn't really care for "Where No Man Has Gone Before" so if it is Mitchell I may enjoy it.
Gary Mitchell makes more sense, especially since they mentioned that it is a "threat from within the Federation", something which Khan never was a part of. Gary was part of Starfleet when he received his powers, and he would be a good personal foil to Kirk. Of course, both Kirk and Mitchell had been in Starfleet longer than they (or at least the current Kirk) have been in the new continuity.
I'm wondering how many movies will pass before they bring in the Borg. With the old continuity being shunted(it's still there, but in a different reality), the "neutering" of the Borg hasn't happened, and they are lurking somewhere in the far reaches of space. Of course, Q would have to make an appearance for that to happen, wouldn't it?