http://www.destructoid.com/neat-adapter-allows-gamecube-controller-use-on-the-wii-u-255393.phtml
It sort of already exists, similar to how a Classic Controller works, all we need is confirmation of how it will work for Smash. It's not unlikely that someone will go the extra mile and make a 3rd-party controller in the exact same shape and style like they do already but without a GCN wire port.
Ahhh yes. Great action.
On a side note looking forward to playing with Mewtwo again on Project M, Nintendo needs to hire these guys.