Personally I'd love a hybrid of the Wii U GamePad and 3DSXL, combining certain aspects from both of those devices.
Here's my attempt at a mockup:
And a side by side with the current GamePad:
The screen size is identical on both, but the display looks bigger on my redesign due to the reduced fat.
Basically, it's a much smaller (and in that case, probably lighter ) version of the GamePad, with the matte finish of the 3DSXL, and a slight button reconfiguration. :awesome:
The control sticks are like the Circle Pad of the 3DS/original GamePad design, but with the grip style from the finalised GamePad.
The Power and TV Control buttons would be moved to a side, and the mic to the side under the screen. There would also be no need for a battery indicator, if it was built-in to the home button:
Everything else is more or less the same, so it's really just about the size and surface material.