Viewtiful Joe would have been awesome, but since he didn't make it this time, he probably won't make it in in later games, since new characters need to either be a revived relic (like ROB and Pit) or have a screaming fan group that will buy the game for the sole reason that their favorite character is in the game (like Snake, and somewhat Sonic and Olimar), and the Viewtiful Joe games were out too recently to be old, but too late to have a screaming fan mass.
But seriously, it'd be awesome to have him, since all he normally does it beat stuff up, he'd have alot of move to work with already, unlike ROB and Sonic, who simply re-use the same sort of moves (rockets and general spinning make up half their movesets).