If there was any doubt about Union Cross ending soon, here you go.KHInsider said:
That's basically screaming that's the thing Nomura plans for the next saga. So really, KH won't end in 3. But the next saga will start with UX somehow.
If I were to guess who'll be put into the bus next saga among the heroes, certainly Roxas, Xion, Namine, and possibly Axel. Axel is a little bit on the middle ground since he actually chose to become a wielder himself. The former 3 are the ones who's really likely to just be cameos, since the concept of 'Nobodies' seemed contained in the Xehanort saga. Unless of course, the concept will have to do with what happened to Strelitzia. Then there's also what people have said on what the former 3 really want, a peaceful life. So I doubt they'll be involved all that much, especially Xion since she's really not a Nobody. Roxas and Namine, may be involved because of their existence as Nobodies. But I think that's probably just going to be it.
Terra and Aqua, on the other hand, might actually be the ones who's going to be used more early on the next saga. It's by virtue of being in the same line of wielders under presumably, Luxu's school, and probably the only living ones too since Xehanort and Eraqus might really be killed off for real in 3.
Also, they're Ven's current friends. They're really going to be involved with him at some point. But I think they'll be irrelevant in the latter part of the next saga though, since it's probably going to focus more on the MoM, the Foretellers+Luxu, the Union Leaders, and other new characters.
With Ventus, there's no doubt about it, since he's already in Union Cross.