Some headcannons;
Wario has taken up gardening in his spare time because fresh, home-grown garlic is infinitely better than any cheap store bought stuff.
Nintendo is actively keeping Waluigi's unfathomable true power sealed away by keeping him as a side character instead of giving him his own game. If he is to be given his own game, he will be unsealed, and there will be no stopping him.
Luigi plays DOOM. A lot.
Toad has a few terabytes worth of video footage- with sound- of Mario's most embarrassing moments. He could auction it off for billions of coins at any given time, but actively chooses not to. This is not out of goodwill, though. This is a bargaining chip in case things go south.
Mario has a collection of the strangest objects he's found during his adventures.
Yoshi is strictly kept away from Luigi's insect collection because if he isn't; there will be no more insect collection.
ooh good headcanons!
the Wario one is kinda understandable, since he has been shown to garden in Game & Watch Gallery 4's Fire Attack selection screen animation, though i personally think hr does a mix of gardening and just buying it. since (unless he uses any castle/manor/whatever other plot of land that he owns that isn't just his house-) he... probably doesn't have THAT much space to grow too much.
as for the Waluigi one... honestly with how many random ass powers that man has (and that i want to write how he figured out he had them-) if he was real, i would be afraid of him as well lol-
i like the Mario one kinda? it would probably be the reason Wario and him aren't TOO bad w/ each other past Mario Land 2-
(god i need to work on tossing out some of my own in general sometime aaa-)