Here's an idea. How about NO Khan in the sequel. Let's do something NEW, people!
Yeesh.
Totally.
There is this expectation amongst some (those IMO who fundamentally don't quite 'get' Star Trek and want to force it into the Hollywood mold) that every hero of the silver screen needs a nemesis. I disagree - Kirk does not need a nemesis. Even if he did, Khan ain't it.
Putting Khan in Trek 12 is a terrible idea. It would show that the writers are both bankrupt of ideas and that they do not understand or respect the essence of the show.