Give me a few minutes to come up with something.... However, why not make something very simple first? How about a protection film that they do with cell phones and other devices. While the DS, 3DS had some 3rd party companies make them I rarely saw them in the stores. I know a lot of people probably would want to protect their gamepad screen from being scratched.

Something like this that uses "helicopter blade technology"
http://www.shieldskins.com/

Ps. I can probably actually give someone who gives a great idea a % of the profits (not positive) but you have to join

http://www.quirky.com/social_invitations/2529

How about when it is charging you display Mario with a mushroom going towards him and when it is fully charged he becomes Super Mario. It would be a mix up from the basic battery charge level that you get on all devices. You would have to get Nintendo's blessing on that. Most of my ideas happen to include Nintendo property. I don't think you are in position to pay any royalties are you? Not sure how that all works...

How about a better idea.... Make faceplates for the Wii U controller like the 360 had with the system.   Have better grip or something on the bottom and Nintendo IP on the front.  Yea.. not a very good idea...  I need more brainstorming.