If you're a fan of the GameCube controller and lament the lack of love on the Wii U, it appears as if this adapter is your answer. It appears as if this unofficial piece of hardware will allow you to utilize compatibility options with all Wii U games supported by Wii Classic Controllers and the Wii Classic Controller Pro, as well as customization for the A, B, X, Y, L and R buttons. Keep in mind that does not include Wii-mote or GamePad enabled games.
I've used adapters like these for years, such as a PlayStation to Xbox/Xbox 360 adapter for platformers, and so on. I really enjoy the ability to play games with any control method -- something the PC comes in handy for. Keep in mind that at least one Amazon product review notes that while it does work with WaveBirds, it drains the battery pretty quickly.
GameCube Controller Adapter [Amazon via Nintendo Life]
via destructoid http://www.destructoid.com/neat-adapter-allows-gamecube-controller-use-on-the-wii-u-255393.phtml