They did "Where No Man Has Gone Before" in the current comic, and Mitchell is dead. Not that death is a major handicap in Trek, but the movie would have to address those events and not screech to a halt. Khan's backstory was a little easier to summarize.
It's got to be someone in Starfleet, and probably high ranking. Captain Garth was someone Kirk looked up to, so that would add a real dimension if he has to take him down.