I personally think people who say Mewtwo, Ice Climbers, Sheik, and G&W will be cut are stupid. The reason is that there is no reason to get rid of characters with original movesets. The interview with the Zelda director pretty much confirmed Sheik and Gannondorf (Not there was any doubt...unless you're stupid i guess). Mewtwo is pretty much the only badass Pokemon that can be added besides Lucario. Lucario is nothing like Mewtwo so he couldn't replace him. Mewtwo has to be in Brawl. He is one of the most recognized Pokemon from the first 151. He is badass. He will be in Brawl. G&W was just fan service. But what is Brawl but a giant ball of fanservice on a disk. He will stay. Ice Climbers were selected, because Sakurai saw potential in them. He also loves Retro characters. That's what they are. They will stay, because honestly, I can't see Sakurai wasting that potential he saw in the first place.
The ones who will probably be cut are Dr. Mario, Pichu, Marth (Since I see Ike as Marth with an improved moveset and better recovery), Roy (Because Ike already pretty much has Roy's firey B attack but improved so he is probably out), Falco (He does have a chance if they can give him a new moveset. Maybe give him the bazooka from Assault) and Young Link (I don't really see Sakurai putting WW Link in the game. He would just take the place of a more original character. There only needs to be one Link)