I believe the problem isn't the obscure characters, so much as the licensing nightmare it would be to get the US rights to them all for this game. Far more trouble than it's worth. Personally, I wouldn't mind Nintendo adapting it so that it's got a billion forgotten Nintendo characters (Triclyde!) as support characters and features Nintendo locations.
I mean, hell, it's already got the SMB coin sound in there - why not just go all the way?