I think this will be the game to touch base and showcase all the different kinds of fun.
-Obviously more characters (more 3rd party); and also more characters in game than just 4
-Ability to combine Power-Ups
-Mini-Games
-More than one finishing move (I swear, some of them are useless in Brawl)
-More in-depth campaign mode, touch base with each characters' world, (Speed = Sonic, Exploration = Link, etc)
-Extensive online; even to showcase the MMO aspect of single player mode,
-Even Achievement/Trophy system