Not the most... expected combination, but the original Doom with a sugary sweet Super Mario 64 filling works surprisingly well, all things considered.
Doom: The Golden Souls is a mod heavily inspired by Super Mario 64. It's been in development for quite some time, but version 1.0 just butt-stomped onto the surface of the goomba head that is our Internet. Admittedly, these cotton candy cloud landscapes are overrun with demons -- not koopas and goombas -- but that's kinda the appeal.
That said, this isn't just Doom wearing Mario's overalls. Gameplay elements seep in as well, and the result is an interesting concoction that's part-old-school-FPS, part-platformer:
"This mod has got alot of different gameplay styles: You may have to simply reach the golden soul in a level, while in another you'll have to rush through an Athletic level, avoiding obstacles and such, you may have to help some NPCs in order to reach your goal, or even collect 8 red coins to get the Soul."
Also, this is delightful: