I would make NHL game and have all possible coaching options on the screen.
In the same vein: a RPG where you issue commands to your warriors by drawing on the touchscreen, which is your "coach chalkboard", so to speak. You then watch the outcome on the big screen.
In this mockup, Maxim (red) will rush toward the boss and attack him with his sword. If the boss counterattacks, Maxim will surely be violently thrown back, so let's make sure that Selan (green) moves out of the way. When she's safe, she will cast a spell to revive Guy. While the boss deals with Maxim, Artea (blue) will sneak behind him then shoot an arrow at his back, striking his weak spot for maximum damage.
And here's something for non-gamers that I posted
in a previous thread: an interior design software. Design you living room and browse the furniture catalog on the GamePad, see the result on the big screen.
Here are two mockups:
Thanks to a partnership with Ikea and the like, you could use furniture that exist in the real world.
Also, thanks to the Panorama View feature, you could tour your virtual apartment.
Sure, there might be similar apps for tablets, but with the Wii U you can marvel at your creation on your HDTV and show it to friends through the Nintedo Network.
As I've already said in yet another thread, I see a lot of potential in a system that links a tablet (on which you can create by drawing, drag & dropping, etc) and a TV screen (on which you can witness the result of your creative efforts in HD). The GamePad is a good fit for games were the player has to create something, like Art Academy; a game in the same vein as LittleBigPlanet could be great too; one could also imagine an "animation studio" (create your own movies); or a LEGO game focusing on construction.