Just to let you guys know, I'm starting my own Kingdom Hearts story as well. It's going to be called "The Shattered Mirror" and based around Riku when he finds himself in the secret place a few days after he returns and sees a dark presence there holding a mirror. He realises that it's a mirror to take the world into darkness and tries to take it from him. However, the person breaks the mirror and Riku wakes up to find himself in another world, where he is the grand overlord of a massive empire of the worlds.
What do you think?
And to keep on topic, I'm looking forward to the Underworld part with Jemxas, Claxil and Xidon.