If they're gonna go out of their way to get old controllers to work, it'd probably be for the GameCube Adapter since it's the most essential for competitive Smash, especially given Nintendo's strides towards legitimizing Smash 4 in the FGC.
I´ll rather they release a wired Pro Controller. And make USB Gamecube controllers for those that want the option. I also want to use a USB controller for Pokken, if possible. Not the Pad they are releasing, that looks uncomfortable.