Personally i think the new Star Trek film was reasonabily good. i think the story couldve been a bit tighter, the thing that made it work was Kirk and Spock.
i dont think Khan should return, mainly because hes been done already, in what many fans would consider a definitive iteration of the character. why do it again? theres so many different villains and stories they could do rather than this.
My guess would be that they are trying to stay away from regular star trek as much as possible. Borg, would be one example. Although first Contact was great!
First Contact was awesome; but the same thing goes for that really, the Borg have been done to death in the tv series. they are perhaps a more relevant villain, in the sense Khan is just a guy who hates Kirk. the Borg are philosphically against everything the Federation stands for. they are a force of nature. and i would love to see an origin story with them. Khan not so much; people will love it for nostalgia. i doubt it will be that good.