You know, I never was really big on Waluigi back when Smash Ultimate speculation started, I was just more concerned by the fact he's never had a major appearance... and sorry, but I don't count a dancing stint back in Mario DDR. But honestly, with Piranha Plant taking the spot before him, Toad/Captain Toad, or even the third party Geno, I've warmed up to him. He's certainly got more personality than a random plant. If we could get him, and a Toad, most likely the captain, I think Mario's cast would be rounded nicely.
As for getting Waluigi more main line appearances, I don't think it should happen... at least in a Mario title. There are two places I think I'd prefer to see Waluigi more:
Wario Land: I want Wario Land to come back, and I want it to tackle the idea of multiplayer. So who'd be Wario's player 2? Who do you think? Waluigi would likely have slightly different abilities than Wario, most likely not having the shoulder bash, but having something that works similar to it to make up for that. It'd just make sense for the wacky anti-Mario duo to go on treasure hunts together.
Luigi's Mansion: Somehow less likely to happen since Luigi has King Boo as his nemesis, but I think there's a ways of making it work. Say... Waluigi, in some weird antics, accidentally frees King Boo from his prison while breaking into E. Gadd's lab hoping for a score of something valuable. At first terrified, the Boo leader coaxes Waluigi to assist him, partly due to fear, and partly due to promising him riches, as well as the chance to take out Luigi. From there, Waluigi takes an old Poltergust, and uses it to do battle with Luigi whenever he encounters him.
It'd make more sense to me for Waluigi to make major appearances in these games more than main Mario titles, aside from maybe the RPGs, and these would be a great way of getting him into a spotlight of sorts.