Poke through the list of "action RPGs"? http://www.gamefaqs.com/ps2/list-73
Some asked after games similar to KH2 recently. Still an open question on KH2's Answers page. http://www.gamefaqs.com/ps2/915410-kingdom-hearts-ii/answers?qid=252111
A version of KH Re:Chain of Memories was released in North America a while back. Fills the between space. The gameplay is different -- some throw in the towel before figuring it out, it's that different -- but (arguably) very satisfying. Riku's story is filled out there.
You could also try asking over at the PS2 board, though I'd search topics there first.
Guides & other contributions: http://www.esque.com/slr/gamefaqs/ FFX, FFXII, KH, KH Re:CoM, KH2, Okage: Shadow King
The next closest series I can think of [and bare with me on this here as KH is very original] is the Tales series. They are somewhat similar but playing Tales reminded me of KH somehow. Ex - Tales of Vesperia, Tales of Symphonia.
Well for xbox 360 there isnt very many Fast-paced games besides FFXIII which is a very linear game. i love KH series and i enjoy Lost Odyssey and Blue Dragon quite a lot. and if you want a VERY EXPLORATIVE game, you can check out Elder Scrolls: Oblivion. they have a new elder scrolls coming out this year as well, can't wait! You could also attempt to get Star Wars:Knights of the Old Republic. its a regular Xbox game but will work on your 360.
That was my answer to the question link that falconesque posted. in that case the peson owned a 360 Wii and ps2
If you can't see the bright side of life, polish the dull side.--- Join The Army, Visit exotic places, meet strange people, then kill them.
Nier has similar gameplay, but a much darker mood and backstories for the main characters. However it has multiple endings that you can only unlock after completing the game so it has some replay value. The setting is also very nice and the soundtrack is absolutely beautiful. I'd recommend it to anyone here.
"Well then Metallica's S+M can come round to mine and we'll have buttered scones for tea" 100% lolworthy.