Does nobody realize that they're going to tweak even the mainstay characters with existing movesets? Just like they changed Link's smash to have a second swing, added electricity to Pikachu's downsmash, and so forth, we'll see some of those sorts of changes in Brawl. No character will have exactly the same moveset as Melee.
Fox will definitely be back, and the shine is going to stay, though it might not spike any more.
I think Ganondorf should return as the Wind Waker version, with two swords. Having him be cel-shaded in the midst of all the high-detail models (like Game&Watch is 2d amidst a sea of 3d) would be pretty interesting, though it seems unlikely.
I would not mind seeing Young Link or Pikachu go. Falco can go, too, but I'll miss him because I like the way his attacks flow better than I like Fox's, but then again, that'll all change in Brawl anyway.
I would rather see Mewtwo in the new version than even Jigglypuff (even though Jiggs is probably my best character) because Mewtwo's just got the right style. Hopefully his moveset tweak will make him better, though. Jigglypuff can come back, too, though - her moveset is pretty cool and she's surprisingly powerful in Melee. (Yes, I call Jigglypuff "her". Go figure.)
Ness should come back because Earthbound is cool and different, plus that new Earthbound GBA game came out in Japan this past year - again, tweaking to make him a little better would be nice.
I don't care about Luigi, but he probably has to return just because of who he is. If we're lucky, he'll get a Poltergust or something and branch out a bit more.
Ice Climbers and Game & Watch... well, they're not major franchises any more, but they're interesting characters at least. I could see them going either way.
Dr. Mario should get the boot. I don't see how they can branch him out from Mario enough to make him more worthwhile than other characters on this list.
I'd also lose Pichu before the others - by definition, he just has to be similar to Pikachu. And that's just a waste.
Young Link, too, can go, but probably won't. Honestly, though, Link's moves aren't that different from youth to adult to make a believable moveset for Young Link that's at all different from adult link.
Captain Falcon should stay. He's fine as he is.
Bowser - well, we have a new Mario villain in Wario, but on the other hand, Bowser is kind of important... Maybe if he played a bit better. I just think there are unnecessarily many Mario characters already.
Marth and Roy - well, if their movesets were differentiated, maybe. I don't care which one goes, but it's probably a waste to have two of them. I'd rather see some female Fire Emblem character (don't know who there is to choose from), which might make for good reason to replace...
Peach - I don't think she's necessary. Though she did get that DS platformer recently. Could be worse, I guess, and the game definitely needs some women to make a gender balance here.
Speaking of gender balance, there's the case of Zelda/Sheik. If she stays, she should still transform, but would have to make the two characters both playable so there's a reason to transform back into Zelda on occasion. If that's too difficult, I could see a Twilight Princess sword-wielding Zelda that doesn't transform working out OK, with or without a separate Sheik.
(oh, and this is the wrong topic, but Krystal would also be a nice case of adding gender balance, I suppose, if Peach or Sheik were dropped.)