Jeff, Paula and Poo from EarthBound! Come on! Mother 3 was just released, this would be a great way to celebrate. They NEED these three in this game.
The idea of Advance Wars COs in this game is also awesome. Even if it was just Andy, Max and Sami, they are all different enough to be more than satisfying.
As for third party support...
Farthest stretch ever, but I'd like to see the Ouendan boys in this game. Their cheering stances would be great attacks...
Sega's Feel the Magic/Rub Rabbits people would be fun to play as too. Heart eye beams and what not.
Phoenix Wright and Maya would also be pretty cool. You could have Phoenix using files and briefcases while Maya conjures spirits and transforms.