I didn't even know that Waluigi existed until the Mario Party games came out. I would be interested in learning a bit more about him. I think he would have to be coupled with another character first in order to make it work. He just isn't popular enough to be able to stand alone. Maybe instead of Mario and Luigi it could be Mario and Waluigi? or Luigi and Waluigi? I bet the second would work out pretty well.
Honestly I don't think Waluigi deserves his own standalone game. His character is too one-dimensional and boring. He needs to be developed more and become more popular before he gets his own game. Characters like Mario and Luigi and well known, so they can get their own game because they are household names, but Waluigi inst. It just wouldn't work out.
Although Nintendo did indeed have massive successes with a great series of Donkey Kong and Yoshi games, and some lesser-but-still-substantial success with a rather fun series of Wario games: they've also been struggling with launching a successful stand-alone Luigi game.
Given that Waluigi is less well known and popular than even Luigi and is as such an even riskier gamble than Luigi as a the star of a stand alone game: I highly doubt that this would happen.
I would place my bets on Toad, Bowser, and Peach each getting their own games before the likes of Waluigi would--and also doubt that any of those three would get their own games.
I wouldn't be that big on a Waluigi stand alone. Mainly because I felt like he was tacked on to be "Bad" Luigi and because they already had Wario. It just felt like that needed to couple them up together, even though there's no back-story (He just showed up one day to play tennis...That was it. Mario Tennis is when he came in.) So, I don't see many fans clamoring for more Waluigi. (It's like Tingle all over again from Legend of Zelda.)
I think a Waluigi stand alone would be great. I think that him and Daisy are the only ones that are missing their own games. I don't know what they would do for Daisy but the possibilities are endless for Waluigi. Nobody wanted a Wario game before it came out and that was a huge success. The same thing could happen with this new generation of Nintendoheads and Waluigi.
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!
Over and beyond the lack of appeal to the general audience (I don't think he's popular enough yet to warrant his own game), I also honestly don't think that Waluigi's character is fully defined and expanded enough yet to warrant his own game. Then again, I suppose that such a game would be the best place to further define and expand the character.
I also always thought that it would make sense to just invert the 'L' in Luigi's name to match Wario's naming scheme rather than just adding "Wa" to the start of "Luigi"--but then how would one pronounce that? "Gamma-uigi?" (Perhaps someone who speaks Greek would know.)