This is amazing. I never thought of this. All those years in SMB3 my friends and I played against each other in versus. In NSMBWii, the funnest part was sabotaging each other's attempts at jumps at the right time. The Gamepad officially adds in a way to fuck with your friends while you play, giving a true versus mode. As your friend plays, you are the asshole that puts random blocks everywhere to try and stop him.
If Nintendo had its shit together, NSMBWiiU would have a separate mode where you try and sabotage people, with online playability, and you get a ranking based on how well you do at trying to sabotage people. The person you play against also has a ranking based on how well he/she is at surviving sabotage. So, each player has 2 ratings, one for the saboteur role, and one for being Mario. Your rating goes up or down based on how difficult your opponent was compared to your current score (like chess), and you can thus seek out the best saboteurs to test your luck against their skill.
Then, the game should have the ability to upload video recordings for others to watch so everyone can see the epic saboteur vs. Mario battles unfold.