—MASTER HAND—
Imagine Master Hand finally being playable, only, it could be solely played with the Wii U Gamepad. As such, the player controlling Master Hand would not necessarily fight AS Master Hand (though you could by issuing commands), but use the touchscreen to interact with the stage. Master Hand in this scenario would be less a boss and more a GM. Just as so many tabletop games are governed, you have the players (often times four) and the Game Master who regulates the stage. The player using the Wii U Gamepad could add items, platforms, obstacles, foes, and perhaps even change atmospheric and conditional effects like day/night and gravity at the tap of a finger. Even literally speaking, the Wii U Gamepad player (Player 5) would be using his or her hand to tap on and interact with their personal screen of the smash stage. Players would fight each other on the television and the Master Hand player would sit in their own world, playing tricks or favoring certain players. Definitely not tournament viable, but an addition I see very likely.
Here's a quick mockup of how it could function: