Zachary Quinto has said the Star Trek cast all signed up for three movies, so the next one is the end of the contract. Since those characters go on forever, do you really expect this to be the last one for your team, or for you as Sulu?If pay any attention to cast interviews and the convention circuit, you can tell who the enthusiastic cast members are and who would rather move on as quickly as possible. In this case the "likely to return for 4th film" is Simon Pegg (Scotty), Karl Urban (McCoy), John Cho (Sulu), Zachary Quinto (Spock). Anton Yelchin (Chekov) comes across as ambivalent but game if the others are willing. However it is very clear that Chris Pine (Kirk) and Zoe Saldana (Uhura) are desperate to move on and would happily skip the third film if allowed.
The reason I hesitate is just wondering if we could get everyone. It seems to be an all or nothing proposition. I want to say, for me, sure, let’s do another. I enjoy that group. I enjoy what Star Trek films do in general and say in general. And I will say one of the pleasures of having been in Star Trek movies has been seeing what effect they have in society, like astronauts saying, “I kind of got into this line of work because my imagination was sparked from watching ‘Star Trek.’” That is very cool. What I’m trying to say is, I just stand by that product. Getting a whole cast together, that’s tough.
I understand, if one of the cast is unavailable, there’s no point having most of you and recasting one role.
I would have to think about that. I guess you’d have to see when you got there, but I would like to keep it all together.
In Zoe's case her star is still rising and she has the Avatar sequels to look forward to so doesn't really need the money or attention. She is gone and the cost to get her to come back is probably beyond Trek's budget. As for Chris Pine, I think he doesn't hate Trek, he just doesn't care about it either. Its just a job and a job that he worries could lead him to being typecast. However, he has no franchise waiting in the wings to replace Trek and his movie successes has been uneven at best. If by 2017 his movie career is only remaining afloat because of Trek, he might be talked into returning for a 4th film. So to me the real question becomes - can a fourth Trek continue without Uhura? Sure why not, her character has never been that important to the films. Replacing Kirk would be tricker. Not because of Pine (my opinion on his acting falls under "meh") but because of it could be jarring for audiences. I wouldn't mind seeing Paramount et al. try as a strong enough replacement actor can make audiences not care about the cast change. Unless they have a better plan B in reserve. Like say a TV series.