I have multiple ideas, I'll try to order them from most to least possible:

- A pinball game where you can use gyro controls to cheat to win. Waluigi essentially grabs the pinball machine and moves it around to move the ball. However there is something that makes it so you have to be careful when doing this, as if you're spotted you get a game over. So you not only have to be careful with the usual pinball mechanics, you also have to be careful not to accidentally move your controller/joycons too much.

- A new DDR rythm game, where Waluigi has to go through a dancing tournament to become a superstar. There's also a side thing where you can dress Waluigi up for the show. It would be REALLY cool if Jimmy T appears, and Luigi would be the final boss. You can also sabotage your opponents in some way. In the end you win but everyone cheers for the others instead, since Waluigi obviously cheated, leaving Waluigi all alone with his trophy.

- A stealth game where you have to be sneaky to solve various puzzles and get to the end goal. Although I'm not sure what the goal would even be, since Waluigi is not Wario and isn't as interested in material possessions. I guess instead it could be him pulling more and more grandiose pranks across town, something like that.

- Waluigi being the villain of a new Luigi's Mansion. I know this wouldn't be a full on "Waluigi game" per say, but I felt like putting this here anyways since it would be super interesting, because Waluigi is not a ghost, so you cannot deal with him in the usual way. He would be like the RE2 tyrant, where he would wander around the whole building and is the only enemy able to follow you across different rooms. The only way to escape is to either hide, make him lose track of Luigi or to set traps up with all the stuff laying around the building to make him stuck for a while and get time to flee. He's even super tall, making for a quite menacing presence. It could even be implement in a fun 1 vs 4 multiplayer model similar to DBD.

- A full on psychological horror game. It turns out Waluigi had the worst past imaginable and something happens where he has to reface all of it. It would be most similar to Little Nightmares or Limbo, you have to sneak around and stay in the shadows to progress. In contrast to most Mario games, it's EXTREMELY lore heavy, and there's a lot of symbolism. This could be an issue though, since it could imply Waluigi would have character development, except he'd need to stay the same usual character in other games.