Yeah, when Smash Bros was first created the idea of fighting as Nintendo Characters was great...and when you realized it played more like a Platformer than traditional fighter you realized that the designers could spend the time to make the characters feel like their specific game.
And that is what the original Smash Brothers Did...with a few exceptions here and there.
But with Brawl, many of the characters have seemed to lose that appeal of "feeling" like their original games...and only a few characters actually broke the mold of Smash Brothers Formula-ed characters, and those are the harder, but also more enjoyable characters to play as.
Those are the: Pokemon Trainer, Olimar, Zero Suit Samus (to some degree, but not totally) and Solid Snake, and Yoshi.
Everyone else seems to have a Dash or Projectile attack, an up recovery move, and either some sort of counter attack move, or a Projectile Reflector move.
I mean the characters themselves are varied, and very cool, but their moves not so much.
I don't know if adding more 3rd party characters and such would of allowed for the variety to expand or if those characters would have just felt the same.
I do know that I had some great ideas for a unique Bomberman character and for that reason alone I think there could be one more game in the franchise, because if I can come up with good ideas for characters, the developers should be able to.