I honestly don't think anyone should be removed. Modifications should definitely be made, such as making Mewtwo a lot better, like everyone said, but noone should really be removed. I mean, they didn't remove anyone from SSB to SSBM, so why would they remove anyone from SSBM to SSBB? Also, taking away characters could really lose some fanbase from people, which wouldn't be good for sales.
As I said, what really should be done instead of removing characters is updating them. Work with the clone characters to give them their own moveset, or at least Luigify them. Tweak with some of the characters' speed, recovery, etc., like they did for Melee. Then maybe find some way to add in extra attacks, if they have any extra time.
Main thing is, I'd rather them work on perfecting the characters, rather than them deciding who to take out.