Nice ideas. I was thinking of having Xana use both Heartless and his minions, but not combined into one being. For example, Xana uses the Heartless to attack the real world, while he uses his own robots when inside Lyoko.
I've sorted the worlds into order:
Full Metal Alchemist world
Ranma 1/2 world
Destani (world in K.H before Destiny Islands), and End of the World mixed.
If we can keep everyone in the RP, this'll work out.