This might be one of the few areas in which Disney owning Marvel would be a good thing. Riku/Venom/Terra teamup would be pretty cool. Can't imagine what the corresponding keyblade would look like for a Marvel world.
I really liked the command wheel and command melding from BBS. Honestly, the mp system from KH 1 and 2 were shit. You ran out mp right away... and for some reason, you had to really earn more mp to equal 1 bar after the first two bars were filled in order to get one more bar. In BBS, commands/magic were on an appropriate cool-down rate instead of something you used up in a few weak moves and spent forever whacking enemies to try to get back.
I also liked how certain commands could only be made/unlocked and used by specific characters, based on their element/fortes. (if you don't know what I'm talking about, look here)
I also really liked the idea of Dimensional Link, where your character 'borrows' the powers/play style of a character they linked with (including intriguing ones like D. Link with Maleficent as Terra). I enjoyed this much better than the summon system previous employed. This also kinda reminds me of Navi chips and navi-syncros from MMBN. Imitation is the highest form of flattery, wot.
Mmm, maybe they will relent and include more FF characters. They've been careful to keep it to FF 7 and 8 characters up till now. But now I really want to play duodecim again and my vita is broke. If they do include more FF characters, my money would be on them including FF13 characters. I can easily imagine Sora talking to Hope, and wonder what an interaction between Lighting and Aqua would be like.
That being said, I hope they undo the tragedy of BBS at least midway through the game and not at the end, and allow you to play as (or with) Terra, Aqua, and Ventus at some point. A trio of such potential and bright outlook... and then Xehanort fucks everything up and all three are isolated and imprisoned in one way or another.