Travis Touchdown, nuff said. As for third party support, Raiden from MGS would be a logical addition, maybe Grey Fox as well.
As for off the wall shit that's never going to happen, I want to play as Guts from Berserk and Spike from Cowboy Bebop... as well as Commander Sheppard, why?, because Sheppard is the Chuck Noris of video games. Everyone in his universe loves him for no adequately explained reason, despite the fact that he reacts to the idea of being wrong about something like a vampire reacts to a UV lit cross smelling of garlic, and has no real personality to speak of.
He's also inexplicably successful at everything he does. Regardless of the fact that all of the known universe has thrown their entire army at something and it has refused to go away, Sheppard can basically walk in and shoot it a few times before mashing some buttons and it will undoubtedly explode.
I've started to wonder if his enemies have all been payed off to die to make him look good to whichever crew mate he's fucking this week.
But I digress, in fact this entire post has been one big digression. I can't really see very many ways Smash Bros. could be improved, it works fine the way it is, and aside from the rosters being filled with increasingly obscure characters there's really not much else to do with it...
But Batman should be in it... because he's motherfucking Batman, Batman is like the Batman of the Batman universe(Also The Exile and Darth Revan from KOTOR I & II)