As someone who has some pretty sweet internet and enjoys multiplayer, I'm hoping we get a mix of Mirage Arena and Days' multiplayer. I really like the idea of playing co-op with friends and missions, and the only reason i feel it didn't fair so well on Days was because of how Days as a single-player game was already so repetitive, that the multiplayer aspect was even more repetition - plus, no wifi. I don't think KH3 will or ever should pull a RE5 were it's co-op so you can bro out, but as an addition feature? Yeah, that'd be cool.
But, what I'm hoping for most is Mirage Arena with wifi. I wanna play against people on Command Board and try and outwit people, and then do some good ol' Kingdom Karts Rumble Racing with others because I actually really love Rumble Racing (possible spinoff please fund it SE). A VS Arena/taking on Arena Missions with friends would be cool too I guess but I'm mostly just here for the multiplayer on that sweet minigames. Plus, as I've learned with Dragon Ball Xenoverse, if I am ever underleveled, I can tag along on a mission with normal/overleveled people and let them take the damage while I come in like a wrecking ball.