Batch is guaranteed Boxoffice.
By the time 2015 rolls around, Cumberbatch will be a worldwide superstar, not the emerging star he is now. And it's the twin roles of Sherlock and Khan that will have made him who he is.
Paramount will pay good coin to get him back.
Remember what they did with Q in Next Gen? The key to Khan is to make him Kirk's Joker.
Only they'll do it at the end of the third episode, to lead into the 4th. Next ep won't be Khan, they don't want to lean on Cumby like a crutch. It will be the last scene of the movie, the cliffhanger, when we see him.
These are my thoughts exactly.