It's not quite a Nintendo Wii 2 with a touchscreen controller, but Bruno Soares hack to control retro Super Nintendo games on his Android smartphone with a classic SNES controller is pretty impressive all the same. The hack uses everybody's favorite microcontroller, the Arduino, to hook up to an HTC Desire HD.
The pins on the controller are fed into the Arduino, and then there's a key-mapping app on the phone itself which translates the button-presses into inputs the retro games can recognize. That way, you can reprogram the mappings to suit other games.
Best of all, Bruno has released his source code so that you can replicate the mode yourself. All it needs is a tidy external enclosure and it would be reasonably portable, too, though if you're lacking in Arduino skills then you may be better off looking to something like the iControlPad.