Actually...
Nomura said that Sora's story would only be for a few games. The Kingdom Hearts series could still continue without Sora, just with a new hero.
And if you look at it quite techinically, Sora is supposed to be dead already. People aren't supposed to come back from becoming a Heartless. What Sora did was a rare, and almost unheard of occurence. You know how Axel keeps saying that Sora is similar to the unknowns? That's because Sora is not supposed to exist right now, but he does, which contradicts the system. =) Existing yet not existing at the same time? Hmm, now where does that sound familiar?