Ash Ketchum- The guy is just a Poke'mon trainer....come on, man....
Birdo-I just don't like Birdo at all.
Blaziken- It's a cool Poke'mon, but it's tough not picturing it as a C. Falcon clone.
Goombas, Koopas, Hammer Bros. etc.- Generic enemies.
Petey Pirhana- God no. The only way Petey should make it into Brawl is if they bring back the Home Run Contest and have Petey as the replacement for Sandbag.
Raichu- It would end up a clone of Pikachu.
Toad- He has 0% fighting potential.
Characters who shouldn't return IMO:
Pichu- The majority of Smash Bros. fans hate his guts. Plus he's low tier.
G&W- Having him as the retro character was cool, but he seems like just a one time deal to me IMO.
ICs- Same as G&W.
Doc- Character-wise, he's just Mario in a doctor outfit. At least Kid Link was a younger version of adult Link and Pichu was the devolved version of Pikachu.
Marth/Roy-They could have the more recent characters from FE, but I wouldn't mind if they kept Marth and/or Roy.