Ive been long working on a theory that kingdom hearts is a sort of haven for all hearts, be them real or "fakes."
At the end of both Re:Com and 358/2 days, one of the main characters dies. Both of them, (Xion and Riku-Replica), had "fake" hearts. At the end of Re:CoM, Riku-Replica tells Riku that he's not afraid of death, but wonders where his heart will end up. I take it we all know what happens to Xion, but we're never told where their hearts ended up at. If kingdom hearts truly is Nirvana for hearts, I bet that Xion/Riku-Replicas' hearts ended up there. (Well, maybe not Xion's heart, Its inside Soras heart, but I'm pretty sure it applies for Riku Replica.)
I also have a basic idea theory about how KH could actually be the True Darkness, that's why Xehanort wants it so much.
