I think they should only add one 3rd-party character for every 10 first or 2nd party characters added. At 50-52 characters, that would mean I couldn't condone 1 or 2 third party characters. But could Pac-Man be implemented without making him too balloonlike a la Kirby, Purin, and to an extent, Meta-Knight and King DeDeDe? More importantly, beyond the 4 colored ghost shroud thingies, the fruits, and the giant pellets, I simply can't imagine a good moveset for him. It's going to take a lot of creativity, all for a yellow 3/4 spheroid thing.
It's anyone's guess whether a Tekken or Soul Calibur series character would be a good idea since they're already designed for their respective series of fighting games and rarely appear elsewhere. Obviously we'd have to pick one, but I wouldn't look there either.
Honestly, part of me is adamant there should be exactly one new 3rd-party character, with a little subconscious disrespect for those who want 2 or 3 new ones. But if another Namco character has to round it out at 4, I'd have to support the hero of Tales of Symphonia on the grounds that Tales of... is Namco's jurisdiction.