I don't know if a stand-alone game featuring Waluigi would sell, but I could definitely see such a character introduced in a version of Super Smash Bros or another such game with multiple characters. It would have to start slowly, so that momentum for the character could build and demand could force Nintendo into creating a game featuring Waluigi.
It sounds ridiculous, but someone had to dream up Wario to get that ball rolling, and look where Wario is now. He's a star!