Four pitches:
1. One of those shop-owner games starring anime girls (I think it's Receatter--you know, the one with the meme about how it's really a super-intense business sim under the surface), except it would just be Waluigi, and he'd be like a baker or a grocer or something.
2. A point and click adventure game where Waluigi is, like, caught up in a hard-boiled detective story. He doesn't actually say anything other than the usual grunts, groans, and cackles (plus the occasional "Waluigi time!"), but all the other characters act as though he were doing a perfect Phillip Marlowe impression.
3. A "fugitive" sim where Waluigi has escaped prison and has to go on the road, living incognito and doing odd jobs to survive. At each location there is some treasure Waluigi has the option of attempting to steal, but the steps involved with stealing it increase his risk of being recognized and recaptured.
4. Blast Corps 2 starring Waluigi.