To be honest with the plethora of Square games and their involvement with Mario Basketball i could easily see a character coming from a Square Enix franchise if anything.
Perhaps Cloud, Seperoth. Or maybe someone else.
Personally i would love to see Sonic in it, but if he was going to be announced surely they'd drop that bombshell at the new E3 event in ...June?
Wow...doesnt the world feel weird without E3 in may, its thrown me off a mo fo'in treat!