Sonic
Megaman
Pac-Man
Possibly a 4th representative although no idea who
Comments:
I am pretty confident there will be at least 1 Namco representative, its basically a fact, given that they are helping develop the game.
In my opinion it will most likely be Pac-Man as a character from their other fighting games will probably seem a bit off in Smash.
Also I believe Snake might be cut. With the new inclusion of Playstation Allstars it might be harder to get Snake into the game. And I know Sakurai and Kojima are great friends and that it was actually Kojima who wanted Snake in the game since Melee, but I have a weird feeling that Sony might just buy out Snake or do some deal with Kojima (or Konami) about him not being in Smash.
To end it, I think that there will probably be 4 3rd party representatives this time around and so I believe there will probably be one more that I haven't listed, I am just unsure who that could be. Bomberman maybe?