Sonic Adventure, Sonic Adventure 2: Battle, Sonic Heroes, Sonic Advance, Sonic Advance 2, Sonic Advance 3, Sonic Rush, etc... Sure they're all relatively new (and the first few are ports), but so is Path of Radiance (if that's truly the castle in the trailer). Sega also worked on F-Zero GX (awesome game), and are currently making a Wii exclusive Sonic. I wouldn't rule them out that quickly.
To those that would like Knuckles in the game, I have to disagree. As much as I like him, his ability to float endlessly AND climb walls would make him too difficult to knock off and keep off. Sure they could gimp him, but I'd rather him not be present than suck. If I was going to pick three Sonic Characters it'd be Sonic, Tails, and Robotnik (in some kind of mecha contraption).