I remember there was a flash game called Mario & Luigi Wariance I played as a kid. It was a 2D side scroller. You had to get to Peach's Castle, fight Bower (original, I know) but then you'd switch to Wario and Waluigi fighting Popple.
As to answer the question, I think Nintedo doesn't want to include Wario and especially Waluigi in a non-sport or party game unfortunately. And that's a darn shame. But with things like Daisy playing playable in Wonder, maybe they might have a chance ? (I know I said that like 5 times now but still, it's kind of reassuring)