The cast from the first movie (Chris Pine, Zachary Quinto, Karl Urban, Simon Pegg, John Cho, Anton Yelchin, and Zoe Saldana) will be back. Not sure yet if Leonard Nimoy(or even William Shatner this time) will show up though.
Roberto Orci and Alex Kurtzman are currently working on the script of Star Trek 2, and nothing has leaked yet. But there is this deleted scene from the first movie:
Will the Klingons show up in Star Trek 2?
Maybe the Klingons will be the next villains in the movie Star Trek 2!
Anyway don’t forget the release date of Star Trek 2!