@Jarkes
Not to mention, the only mainstream Mario action games Wario has appeared are only Super Mario Land 2, and Super Mario 64 DS. Wario’s barely even a Mario character. (Probably because he was made by a different Nintendo development team then Miyamoto’s EAD team.)
A Wario Ware poster does appear in “Mario & Luigi: SuperStar Saga” BTW, so that’s sort’ve a cameo, and he had a scrapped cameo from that game, as well.
@Shepherd
Would be nice, but Syrup might be too obscure, and forgotten unfortunately… Then again, K. Rool was too, but he had more overwelming demand to return in SSB.
@09MurphyM
Waluigi also appeared as an antagonist in two of the games in “Game & Watch Gallery 4.”
Not sure if that counts as a spinoff, as it’s a compilation of G&W games, but with Mario characters added to the “Modern” modes, and menus to make them more appealing. (Mr. G&W also appears with Mario in said game whenever you unlock a new game. This game came out a year after Melee.)