Doom has two laws, and they are simple:
Eventually, you will be able to play Doom on any electronic device in existence;
Anything that can be made inside Doom, will be made inside Doom.
A new Doom mod, named Doom: The Golden Souls 3, follows the second law, turning Doomguy into a cute purple-armored platformer hero. "Right after celebrating the defeat of the Cyberdemon, Dimensional Rifts appear outta nowhere and Doomguy along with others are transported in a mysterious city called Limbotropolis," the mod description reads. "Will he uncover the truth about this strange new world?"
An important question. And you can see why this world would be pretty strange for Doomguy. For starters, it's mostly green. There's a lot of grass. There are trees. Even flowers. Has Doomguy ever seen a flower? Like, canonically? Does he know what a flower is? (After a moment of reflection, I think Doomguy would probably shoot the flower).
The Golden Souls still has shooting. In the brief teaser trailer, Doomguy has a pistol that shoots chunky pink bullets and a shotgun, though he spends some time punching and kicking familiar Doom enemies, too. But the shooting looks less interesting than the rest of what modder Batandy has concocted here, in what they call "a true collect-a-thon adventure."
I don't think first-person platforming in Doom will have quite the precision of a real Mario game, but it should still feel pretty damn good thanks to Doom's sense of speed. Batandy has also posted about making significant changes to Doom's movement system to give you control in the air and make platforming more fun.
There's a lot of Mario DNA in Golden Souls, like a mission that tasks you with collecting "eight Slayer coins." There are coins all over these sprawling outdoor levels, enemies that look like piranha plants, and Doomguy has some new skills for getting around. Like Mario, he can now climb ledges and long jump. There are checkpoint flags and moving platforms to jump between, like in any good Mario game.
The modder even built in several difficulty options to make platforming less punishing. On easy and normal, you'll lose coins or some health for falling in a pit. Only on hard will a fall mean instant death.
Golden Souls 3 isn't actually Batandy's first Mario-inspired Doom mod. Golden Souls 2 is actually a huge mod, with around 50 platforming levels. Batandy describes those levels as being more linear, while the objective of this new mod is to build something more sandboxy, akin to Mario 64 and Banjo Kazooie. There's a hub world (I guess you'd call it the Doom version of Peach's castle) and Batandy built a chapter system like Mario 64's, where you'll enter the same large map multiple times with different goals to complete.
Golden Souls 3 is still early in development, but you can download a demo from this forum thread, or play through the entirety of Golden Souls 2 while you wait. You'll need a copy of Doom 2 and GZDoom to play it.
Batandy recommends some modest hardware for good performance, but according to the first law of Doom, you'll eventually be able to play Golden Souls 3 on your toaster.