A portion of an IGN interview with co-director, Tai Yasue...

IGN: What's next for Sora and Riku?

TY: We get that a lot. [laughs] We're still thinking about it. I think 3D is really a hint to the future in a way. We developed it as if it were a console game. We made it for the handheld, but for us it was meant to feel like a console game in a way. I think it really took the Kingdom Hearts series to the next level, it feels like Kingdom Hearts II and Birth By Sleep on steroids. It's really insane what you can do. So I'm not really in a position to say what we could next, but... 3D, for us, was the future.