As a Nintendo purist and longtime fan, I don't really like the idea of 3rd party characters... but apparently everyone else does. My issue is that added 3rd party characters might take space away from many obscure but deserving 1st party characters. For example, Lil Mac, Ray 01, and Captain Olimar would make great Smash Bros. characters, and i think they should have priority over 3rd party ones simply because of the fact that they are Nintendo characters. I think if there are 3rd party ones in Brawl, they'd better be **** synonomous with Nintendo consoles, or they don't deserve to be in there. This would be characters like megaman, and Simon Belmont, those who aren't Nintendo, but everybody immediately thinks of Nintendo when they are mentioned. Sonic I guess would be ok, since he's been around so long, and there is major history between him and Nintendo with the whole rivalry thing, for those actually old enough to remember the height of it...
Sora, or whatever other recently popular character you want to insert here, would be a dumb idea for Brawl, since they are neither representative of Nintendo, nor have been around long enough to deserve to have any notable history with Nintendo. That's an opinion I'd hold even if by some small-*** chance such a character is actually confirmed for Brawl.