I agree that a film featuring Mitchell would have to be much more a straight remake of previous material than one featuring Khan.
I'm not so sure. I suppose they could have the character of Gary Mitchell be a friend of Kirk's, and then give Mitchell a plot that has nothing at all to do with the events surrounding WNMHGB.
I'm not saying that I think Gary Mitchell is in this, nor am I saying I necessarily want him to be in this -- HOWEVER, I'm sure they could find a way to make Mitchell interesting even without giving him god-like powers.
I feel the same way about the character of Arne Darvin. Darvin could easily be an antagonist of a trek film in the Abramsverse as part of a story that has nothing at all to do with tribbles. It could be a story about a Klingon spy disguised as a human who has infiltrated the Administrative/Executive branch of the Federation....
...there would be no tribbles at all [sorry -- I couldn't resist]