5. Paper Mario
I know what you're thinking, but this won't just be another palette swap (I'm looking at you, Dr. Mario and Toon Link). Paper Mario can have an entirely different array of powers to his 3D counterpart, being able to dodge into the background and having timed button press attacks to deal extra power. His inclusion also gives us an excuse, maybe even if it's just for the 3DS version, to have a Paper Mario stage which would certainly mix up the action. Also, as the series is an RPG and a number of its items can be borrowed for regular Smash Bros. fights. Paper Mario can also be an in for the main game to steal some new mechanics from the latest outing Paper Mario: Sticker Star. Brawl even had collectable stickers (though nobody cared about them as they were essentially a naff extra collectable compared to the already awesome trophies) so this way the stickers can be used to power up your fighter rather than Brawl's method of restricting this to the Subspace Emissary. Maybe not appealing to those who play 'Final Destination, no items' but the rest of us might get a kick out of it.