The next regular installment in the Kingdom Hearts series will be entirely dependent upon the upcoming Final Fantasy XIII spin-off Final Fantasy Versus XIII, according to creator Tetsuya Nomura.
In the Ultimania Guide for the PSP spin-off Birth by Sleep, Nomura stated that "Kingdom Hearts 3 depends on Final Fantasy XIII Versus." What that means is unknown, though it most-likely means the game will not start production until Final Fantasy Versus XIII is released.
Nomura also released more details on the series, such as two more Kingdom Hearts games that are currently in the planning stages along with Kingdom Hearts 3. He said that he hoped to release one of those games sometime this year, while having one release next year with "multiple main characters."
As for the end of the Kingdom Hearts series and Sora’s story in it, Nomura cleared both up. He stated that Kingdom Hearts 3 will not be the end of the series, simply the end of the struggle against central villain Xehanort. As for Sora, his adventures won’t end with Xehanort either.
"It’s not the case that when Sora’s story is over another hero’s story will begin; Sora will be the hero until the end," stated Nomura.
As for the differences with the upcoming international version of Birth by Sleep and the Japanese version, Nomura mentioned that this version will have a Critical difficulty level, more cutscenes, more songs to the Rhythmic Ice Cream mini game, more Keyblades, more types of enemies and altered battle balance.
