Xios_Urahara

I would put someone that would be able to change their form with their wepon (NOT THE KEYBLADE its over used as it is). They would be the new top dog enemy and when he is defeated you can find a chest or something that transforms your Keyblade to have the powers of his wepon allowing you to change into whatever enemy you have encountered and thats in your Journey/Enemy list (Excluding Bosses). They would be hard as hell to kill like Sephiroth in KH1 they would be able to use strong magic and carry alot of items making the boss fight last for a long time also they would be like Riku using the Light or Darkness to fight for the other side in this case Light to help fight for Darkness.