There are some characters that shouldn't have been put in Melee (Dr. Mario, Mr. Game and Watch, Marth and Roy...), and I would not shed a tear if they were absent in Brawl, but I really doubt Nintendo will cut anybody. Replace, maybe (Ike=Marth/Roy), but totally eliminate... probably not. With each new game in any series, it's all about bigger and better, and no matter how much they add, trimming the roster will always make Melee better in a few ways.
Third-parties will only happen when there's a mutual agreement between Nintendo and the third-party people. Personally, I think non-Nintendo characters period is dumb, but Sonic is someone I really would rather not see. I don't know if it's his personality, or I'm sick and tired of everyone (not you guys, just... you know) not shutting up about wanting him in the game, but... yeah. Besides, if you want to watch those two compete, play M&S At The Olympics, or whatever.