See, here's the thing for me.
appeared in TWO mainline Mario games.
Super Mario Land 2 (a kind of a spin-off in and of itself).
And Super Mario 64 DS (shoe-horned into a remake).
It's not even like the case with
where they brought BACK a subseries for them together. Mario & Wario was like a late SNES title and that's the only time the two have been seen directly against eachother since his creation and he was shoe-horned into SM64DS because of muh expansion considering that he and Luigi literally took the metal and invisible caps are their own power ups....
Since then, Wario's had two significant series to his own.
Wario Land+World, the tales of him as a brute force treasure hunter.
WarioWare, the tale of the company tycoon and the various hijinks of his town that require themed minigames.
as a series has always been a part of Mario. Because I can't think of a single game of his where the final boss wasn't some form of
. Not to mention being brought back as a staple stabled steed through Sunshine, Galaxy 2, and the console New Super titles.