Tony Hawk -- Pro Skater was so important to a lot our childhoods and the playstation, I'm surprised how little attention the series have gotten. I know its pretty much a dead series now, and Tony Hawk probably won't give allow himself to be a character, but it would be pretty awesome. Some reference to the Pro Skater series would be very welcomed.
Izzy Sparks -- Guitar Hero rep. Same reasoning as Tony Hawk. Since he's not that much of an all-star, maybe a Guitar Hero stage could be in?
Eddie Wachowski -- I love SSX. While its almost always been multiplatform, its always been associated with the PS2 a lot.
Prince -- Katamari Damacy. I'd rather have a stage based on this franchise, with the King of the Cosmo doing some weird cosmo magic, causing a time rift or something, making another franchise to appear, and the prince begins to clean up the world.
Dante -- DMC isn't too obscure and its pretty likely we'll see him, despite Capcom's claim that they haven't been approached yet. It seems like they'll be very willing to give SuperBot their library tho.
Amaterasu -- Seems to be getting more Nintendo-based than Sony, but Okami and Amaterasu are still fairly important to the PS2, at least to me. I guess she isn't a playstation all-star, but if Big Daddy is, than the term has got to be pretty broad. For capcom characters, Dante is still much more important.
Dark Cloud rep -- Dark Cloud isn't really that obscure since its a Sony franchise, but Toan and Max just don't seem that iconic. As for Sony's JRPG's, Dark Cloud is my favorite. Max can do more and even ride around in a robot, but Toan seems more iconic.
Zone of the Enders rep -- Never played this game, but my friend tells me he wants a ZotE rep.
Onimusha rep -- Pretty sure Dante would fill the Capcom role, but Onimusha is awesome and I think it should warrant a reference or even a stage.