Heard tell of a "Bowser" "Day". Wow!
The Klutzy Koop - Yowser
I bring you another Mario OC. Yowser is a Koopa that Bowser hired hoping his dangerous clumsiness could stop Mario, so he'd be a recurring boss fight.
Every time he's fought, he's got a different gimmick, whether that's through the use of obstacles in the boss fight arena or weapons he wields against you. You have to dodge the ensuing chaos until he manages to clumsily stun himself using said gimmick—with his horns and spikes, him being temporarily stunned is the only time his weak spot, his long tail, is still and vulnerable.
I was imagining him specifically as a boss for 2d platformers, but I think the idea of turning his attempted attacks around on him could lend itself well to a boss fight in a Paper Mario or Mario & Luigi game.
Of course, eventually it might stop being believable that Bowser would keep him around. Maybe at that point he could get fired and move to Diamond City or something; I think he'd fit in with the wacky hijinks of the WarioWare games.
Detective Rye
Here's a character idea I've had kicking around my head for a while but never got a design for until now. I think the association between murder mysteries and a detective that's undead has fun potential, and Mario's Dry Bones offer a good way to do that.