Kingdom Hearts is my favorite game-serie as well and yes, the features in the game would work out very well. A KH mmo on the other hand, wouldn't do any good for me, since 75% of the KH-franchise is based on it's epic storyline.
For Runes of Magic: it is fun, untill level 25. Then it gets repetetive, boring and you see yourself doing the same quests every day, which isn't worth it.