The problem with adding more and more games to the Settings is that, not only will the book get even more (unnecessarily) bulky, but it's hard to justify doing Kingdom Hearts before more 'canon' games such as Vagrant Story (which is set in Ivalice as well).
The thing about the Keyblade master, whether it be Sora/Riku/whoever in the video games or the base class, it has a lot of quirks that seperate it from the other standard classes. This makes it far more difficult to balance - at any rate, it's certainly not a project I'd be confident tackling!
