Here's the best dang idea ever. I had it earlier, but FoC's thread reminded me.
Basically, it's a cross between a Classic Controller and the WaveBird with a little Wii-Remote thrown in.
It's designed mostly like the Classic Controller, except that it's Bluetooth compatible, and thus can be used without connecting it to a Wii Remote.
Second, it comes with a wireless RF dongle that connects to the GameCube ports, allowing it to sync to the dongle and play GameCube games.
A switch lets you change from "Off" to "Wii" to "GC".
Yeah, it'd probably be a little expensive, but wouldn't that kick butt?

*edit* - yes, the controller has both Bluetooth and RF connectivity, just to clarify. I suppose the dongle attachment could use bluetooth as well, negating the need to include both Bluetooth *and* RF within the controller...