It would be an interesting incorporation.
It definitely would bring in a solid brony audience.

Well they're running out of Disney-themed worlds that could be used, I'm having difficulty imagining any new worlds that would be introduced in KH3

Yeah it would potentially yield new bronies, and vice versa (new KH fans)

Darn it, now I can't help but think that I've gotta come up with a MLP FiM themed keychain (the thing that changes the appearance and abilities of Sora's Keyblade)

Spike should be a summon spell!

Not a bad idea, but what would he do?

Fire-breathing is out of the question, Mushu is a summoning spell and he uses a fire attack =\

Uses a gem based power?

Maybe he could increase your likelihood of obtaining items used for synthesis? (Blaze gems, thunder shards, etc.) idk...

That or he could throw exploding gems or form a gem shield around you. I love where this is going. The item obtaining thing would have been helpful in the First game, those gems were annoying to get.