According to Gamer Within, the Kingdom of Hearts may see some further additions. In a recent interview with Japanese gaming publication Famitsu.com, the Kingdom Hearts series' producer Tetsuya Nomura indicated that he currently considering another title that is neither a sequel, nor a prequel or in-between addition.

He said;
It's difficult to explain it. I'm thinking about how to talk about it [without giving things away], which means that there will be some twists to it. Up until now, the series followed along a [straight] time line, going from Kingdom Hearts to Kingdom Hearts: Chain of Memories and Kingdom Hearts II. But the idea I have right now doesn't follow that flow. However, as long as it inherits the Kingdom Hearts title, it needs to have some connection [to previous installments] in terms of storyline, so it won't be too distant from the other titles. In any case, this is still at a conceptual phase, where I'm thinking about what I want to do.
So the Electronics Entertainment Expo in May and the Tokyo Game Show later this year may have some about this.