Well if it were Khan, where would he be returning from? From exile into space? That could work. Maybe this movie is what would have happened if 'Space Seed' had not.
If it was Mitchell, I suppose he returns from whatever abyss he went to after either Kirk or Spock (can't remember who, although this time Dehner wasn't even there) phasered him in WNMHGB. Although on further reflection, why would he be waiving a gun around if he's got telekinesis? Also his eyes look pretty normal to me.
I too would like a regular space exploration story. I think the broader epic 'movie' format requires that something would already be happening though (ie, it wouldn't be an exterior shot, cue fanfare, begin reading Captain's Log).