12 characters is an absolutely tiny roster size by Smash standards, and not particularly large by any fighting game's standards. Also, I dare you to find room for any new fighters on that roster.
Really, 30 is the trendy number to throw around, but I think even that's a little too low. That's only a little bigger than Melee was. Barring massive, sweeping changes to every fighter (which is probably not going to happen, because honestly most characters can keep most of what they have), I think we might see, like, 48 or 50 or something.
I would like to see some new universal mechanics, though. Stick supers on Y or something, and have four of 'em like special moves. I mean, they're giving everyone resource management and upgraded specials and super modes these days anyways. Make it official.