Khan got more or less what he wanted by being back in the tube asleep with his people safe. Didn't seem to me that he really wanted to destroy the human race or anything- that was just a stated presumption by spock based on Prime Spock's recollections.
It's the Borg next.
Please leave TNG-things alone. New Trek is a return to the Original Series which has been pretty much abandoned
since TNG came on the air in 1987. We've already had the Gorn in the video game, but what about the Tholians? There's lots of stuff from the original series that hasn't been seen since it originally aired!
Besides the Borg have been so overused at this point I think you'd get a lot of groaners in the audience. Was anyone really happy to see them return for Enterprise
I think major confrontation with the Klingons is the next thing up, if they aren't going to revisit something else from the Original Series. The Klinks are the designated antagonists for many TOS episodes and production and maybe a showdown with them is due.