And that's my point, who wouldn't wanna see Kahn again on the big screen? The key would be making him evil enough so that the audience doesn't lose sight of the characters their supposed to love: Kirk and co.
I do think they should tighten up the next movie and make it much more serious, much more Dark Knight in the sense the characters truly look like they don't know what there dealing with if Kahn is to be the lead antagonist.
Maybe Kahn and his followers have on board something like a disease that could wipe out everything in existence from their time period. There are so many possibilities in terms of what you could make Kahn and co. all about this time out.