From what I heard Cruise wants to do more after dead reckoning part 2. My guess is that they wont recast Ethan Hunt, instead he will probably take a different kind of role, like a mentor or recruiter. They sort of teased it a bit in this film. And that whole thing with him training Keri Russell in 3. Also apparently they were setting up Renner to take over as the main character after Ghost Protocol, but Cruise delivered so well that they changed their mind, or something like that.
Eventually you would think Tom wouldnt be able to do the kind of stunts he is doing, but who knows how long might he able to keep this up. It would be cool if they came back around and made a more pure spy thriller like the original (minus the train scene) but I don't know if Cruise would want to do something like that since he seems to enjoy the adrenaline of the stunts.