Since all the old Smash Bros Melee character lists are now plausible, I could name off pretty much anyone and they'd have a chance of making it in. Either as a playable character or assist trophy. They better have like 400 regular trophies in the game though, they've got a bazillion other things in there already, shouldn't skimp on a favorite pasttime of obsessive Melee players.
Actually, I'm half-expecting Krystal as an Assist Trophy. I don't know what she'd do, but the Star Fox representation in the game can't stop at Fox and Falco (and considering their appearance was mucked up for Assault and Command, I'd like to see one team member who looks normal). Maybe Wolf o' Donnell, though again I wouldn't want to see the Assault version of him.
I do expect to see Tails as a playable character though. Can't have Sonic without Tails. Or Robotnik for that matter, which was made plausible after seeing the behemoth King Dedede become a playable character (which I think is awesome, never would have expected him, although his hammer better do major damage). And you know, Tails can fly.
As for Mario characters, we already have most of them. ...hey, how about the Koopa Kids as trophies? Way better than Bowser Jr.
Or Flammie from Secret of Mana. The most unlikely choice of all, but I would freak out in joy if he appeared. Secret of Mana was like Chrono Trigger's little brother.
Regarding Ness possibly not making it into Brawl, the fact that Lucas appeared at all drives me batty in two ways. First, having Ness in Smash Bros. was what made me consider playing EarthBound in the first place. So what if they wanted Lucas in the original game, Ness was the man. Told me that Nintendo acknowledged the existence of EarthBound. Now Lucas is here, who few people will know. If people get to like Lucas, wouldn't that signal more people wanting Mother 3 over here? Having Marth and Roy in Melee did eventually trigger some Fire Emblem games in America. Sin & Punishment EVENTUALLY got released in America, on the Virtual Console (speaking of which, it's high time for Star Fox 2 to appear. It was basically finished after all).
Come on Nintendo, give us Mother 3. If not, then give us Ness back. Hey, you're putting the Snowman song into the game (which I misinterpreted as Winters), just go the whole mile and represent EarthBound more.