Let me start by saying that I don't want Masahiro Sakurai to be in charge of the next Smash Bros. title. Not that I think he's a bad game designer or did a bad job (clearly I don't, if I play his games). Rather, I feel that if Brawl was as far as he could take the series with Nintendo giving him every resource they could and no clear deadline, the series should be handed off to someone else who can take it into a fresher direction than Sakurai could. My choices:
1. Shigeru Miyamoto.... because he's Shigeru Miyamoto. He also pressed Sakurai to focus on multiplayer, something Sakurai clearly did not do. Most people would have a harder time coming up with reasons why that wouldn't be a good idea.
2. Suda51. May be too weird for something as mainstream as Smash Bros., but I think he's certainly talented enough to tackle a project like this. It upsets me that No More Heroes did not perform as well as it should have. I think he has a very sharp game designer's mind. With NMH, he is the only game designer who has put considerable thought into how else the Wii Remote could be used and he created a game that is completely unlike anything else on the Wii. I think that kind of thinking would be perfect for making a series like Smash Bros. feel fresh again.
3. Yoshinori Ono. Who? He's in charge of Street Fighter IV. He would be my absolute personal pick though he's the one I would imagine that most Smash Bros. fans would be opposed to. I think that most would, understandably, fear a more complicated game under Ono, but I think Miyamoto and Nintendo would keep him focused on keeping Smash Bros. accessible so I don't think anyone has anything to worry about in that regard. Also, he pitched SFIV to Capcom with the intent on making it more like Street Fighter II, admitting that SFIII, a game he worked on, was too complicated even for many Street Fighter fans. Working on a sequel to Street Fighter is a big deal and a tremendous amount of pressure is on his shoulders to deliver a sequel worthy of its namesake and from the looks of it, SFIV is set to deliver. He knows fighting games, especially how extremely difficult it is to properly balance one since being off even a few frames of animation could wreck the harmony of the entire game. If nothing else, that's the main thing he has in his favor.
If I had to pick any developer for the game, I'd pick EAD Tokyo. They have 2 games under their belt: Donkey Kong Jungle Beat and Super Mario Galaxy. What's not to like about that?