By the way, the next game will NOT be another COM. Nomura confirmed this.
I don't think Kingdom Hearts is using puppets. Xemnas of his own sound and mind wanted power and to be god-like, if you will. Kingdom Hearts was the only thing that could grant him that. Darkness is not something that can simply be controlled, as clearly proven.
It's possible the World Of Darkness which is Kingdom Hearts was locked away behind that door, ending the Keyblade War. Maybe the Keyblades were created as a fail-safe if the door was ever to be opened again. And then they would choose one with the strongest of hearts to stand against the Darkness.