Normally, I think for a lot of characters, popularity hurts them in the long run, seeing how the fan-favorites are so few and afar between, especially with DLC. But for Waluigi, no. Based on that Smash 4 "just because you try hard" quote, it seems like Sakurai holds something against Waluigi, so I don't think he'll ever be a fighter as long as Sakurai's got a say in who gets in.