I think if Soul Calibur or Tekken got a character, it would have to be Yoshimitsu just because he's silly enough to fit. His character design can be as
silly or as
serious as needed. His moveset already fits Smash Bros. perfectly with him helicoptering around and pogo sticking. The only thing about him I'd take out is his, cough, occasional ritualistic suicide.
Pacman is... well, I really don't like his 3D incarnation with the nose and limbs. Looks like a really dumb Chucky Cheese type mascot to me. Mostly because of the nose. If I were designing him, I'd want to make him essentially an amorphous orb with a mouth that spawns limbs/tentacles/whatever as needed.
And I'm a big supporter of Tales representation, which I'm confident means Lloyd Irving.
Dig Dug is a remarkable fit. Wielding a giant drill and harpoon could be super badass. A pity that he didn't actually
originate on a Nintendo system, not that it's necessary but... it would help.
Klonoa seems pretty well suited to Smash Bros. I didn't enjoy playing his Wii game at all, but he still seems appropriate.
I didn't like the Baiten Kaitos games. Then again, I did play two of them and the first one had a really great and original story twist, so maybe I'm lying. Kalas would be nifty... Nice design. I also wouldn't mind seeing the freaky, awful, terrifying creature known as Guillo from the Origins game.
The wiki page for Namco's games makes it nice and clear what has appeared on Nintendo systems.
http://en.wikipedia.org/wiki/List_of_Namco_games