Pac-Man - Come on, Mario, Sonic, Mega Man, developed by Bandai Namco. Will Sakurai pass this up?
Chrom - Marth is the poster boy veteran, both Roy and Ike are outdated. The only ways I can see Chrom being excluded are if Ike and Roy both return or if another Awakening rep is favored (Lucina, Robin)
Little Mac - Also has retro value, one of Nintendo's unrepresented franchises that had a revival. (just like Kid Icarus)
Mii - Would make for a unique addition, used in plenty other Wii/3DS games, and after WFT I don't think anyone would put this beyond the realm of possibility.
I don't think every veteran will return, like I said it would be strange to me if Roy and Ike are both in. I have very little hope that Pichu or Doc will ever see the light again either. Mewtwo though, along with Lucario, may get their Mega forms included. Of the Brawl cast, I think the least likely to return is ROB.
Since Sakurai dropped that hint of Ridley being a hazard awhile back, I only expect him to show up as a boss (again). Same with K. Rool. I also expect the likes of Waluigi and Isaac to be doomed forever as ATs, and Shulk, Custom Robo, maybe even Paper Mario as well.
I don't expect Palutena to be playable, but I won't be surprised if she is. Sakurai doesn't like to over-represent his own work, remember? After the addition of MK and Dedede, though, it's hard to say if KI will get another rep.