Just finished up beating the game again, and I started thinking...
At the moment it appears that KHIII will be a prequel of some sort. But Square threw in that scene at the of KHII with the message in the bottle, so they could return to Sora, Riku, and Kari, in some way shape, or form, but in my opinion this would be beating a dead horse. IMO KHII did a good job of resolving all the problems, leaving few questions except whats in that god damned bottle, so I think the happy ending should stay.
Do you want to see more of Sora, Riku, and Kari, or do you think that they should let them be.