Honestly, I think Simon Belmont would be the best third-party to go with...BUUUUT Konami's basically ****ed itself in the ass so badly, I don't really want to see them given much in Smash to inflate their ego with.
So as weird as it may sound, I actually think that Knuckles would be a great character to have, especially since there's no way he could be made into a Sonic clone.
On another note, which first-party characters would you want in Smash the most? I personally vote Ridley, King K.Rool and Tyrantrum, but it'd be nice to see who everyone wants out of the Nintendo line up.