I'm pretty sure Waluigi's only appearance in Ware is the amiibo sketch in Gold. Not even any microgames... I have gone on record saying I would like Waluigi to get his own game beforehand, since him suddenly appearing in Wario's spinoffs would be very sudden. That doesn't mean I don't want him in Wario's series though.
I think the best way he could be used in a Land game is to have a "tag-in" mechanic like Donkey Kong Country where you can swap between him and Wario on the fly. His lanky build would offer up a unique moveset compared to Wario, so swapping between them could be used during certain steps in puzzles. If the game were to have reactions, he could even react to them in different ways from Wario to open up more potential.
Ware is a bit tougher I'd say, because there's already a couple of characters that I feel overlap with Waluigi more directly (Jimmy T. and Dr. Crygor), so it may feel redundant. Maybe he could set up his own rival game company out of anger for Wario not inviting him to be part of his own? Then when Wario outperforms him as expected, that's when Waluigi gets the invite to join the crew. Or maybe it could just be a one-off story to explain why he's not in the rest of Ware.
He probably could find the most footing in a completely new Wario title, but I don't know if that's ever happening. I've always been disappointed there was never a Mario & Luigi game where the two Ws begrudingly worked with the more familiar bros. in an adventure than pulls from both the Mario & Wario platformers; feels like there were interesting ideas to explore. I also had an idea for a Wario/Waluigi/Syrup game that was more focused on stealth: pulling off museum heists rather than exploring temples for treasures, where the three of them work as a team that cover each others' weak points. Waluigi is the charismatic thief who does most of the groundwork undetected, Wario was the muscle occasionally needed to break into places, and Syrup would be the techie with all sorts of helpful gadgets. It was just a thought that I didn't develop any further though, since it sounded a little too familiar to another stealth-platforming game...
What was I talking about? Uh, Waluigi in Wario games, right. I would like to eventually see him there, yes!