All I know of Final Fantasy: Unlimited so far comes from the Phase 1 DVD and what’s on FF Compendium, but I’ve noticed some similarities to Kingdom Hearts:
(Not sure how many of these are considered spoilers, so I’m covering all but the first one.)
-The main characters (Ai and Yu/Sora, Riku, and Kairi) leave their world for a place that consists of several very different worlds.
-There’s a Big Bad Thing destroying the worlds (Chaos/Heartless), which is connected to a single person (Earl Tyrant/Ansem).
-There are dark, mysterious, brooding characters from a world that has been destroyed (Squall & Cloud/Kaze & Makenshi) who work with or against the heroes.
-Summons come from the transformed remains of those who were destroyed along with their worlds.
As I said, I’ve only seen a little of FF:U, so these may not all be accurate. I also left out direct Final Fantasy references (Cid, moogles, etc.).
Is there anything else to add?