The GameCube could've done OK if they had changed three key things:
1.) Launch in November 2000 (a year earlier), at least in North America. Reposition certain games like Zelda: Majora's Mask, Perfect Dark, Starcraft (64), Conker's Bad Fur Day, Sin & Punishment, as GameCube launch window titles.
2.) Ditch the Fisher Price lunchbox look for something sleeker/cooler looking. No purple other than maybe a secondary color for Japan, add a properly sized d-pad on the controller and go back to the SNES button layout (sorry Miyamoto).
3.) Have DVD playback but offer it via a DVD remote add-on (this off-sets DVD licensing fees from the main unit).
I think they would've sold more in the range of 40-50 million units had they done this.