Ridiculous and improbable ideas, eh? Lemme come up with a few...
H-Chex Quest (3)
Explosive everything (even standing around too long)
Friend Heart Mechanic + Dream Friends in Doom
Every projectile is a revenant's rocket
Doom level that generates as you play it and never ends at all
The Sky May Be 2: The Imp-God's coming for You
"Doomguy and his Buddies are Furries" mod ("Oh, what's that, you say? Yiff in Hell? If by "yff" you mean "slaughter demons", then I'm already one step ahead of you, buddy.")
Mod where you play as Daisy (The demons thought they could kill Daisy, but little did they know this little bunny's got powers...)
Mod where your weapons are replaced with powerful weapons based on the spadinner era of YTP (never has a bowl of spaghetti taken down a Cyberdemon)
Mod where you can finally summon (or rather, toss in) terrywad enemies to murder the demons instead of murdering you. ("Eat DickFail, hellspawn!")
The floor is super-slippery ice, and if affects everyone
Late-game Terraria weapons for Russian Overkill
Play as (or summon the help of) Minions mod
Strong Bad in Samsara
Crash Bandicoot in Samsara
That's all I can think of for now.
EDIT: Few more ideas 'cause why not
Square (The Adventures of Square) in Samara
TAoS enemies in SchMonsterMixer
Nepeta in Samsara
Pistol replaced with Portal Gun
Your only weapon is a weapon that can change the floor/ceiling values of sectors
Everyone/Everything moves stupidly fast
Everyone/Everything moves painstakingly slow
Everyone/Everything moves like a drunken semi-blind clown
Doomguy slaughters his way through Site 19 (H-Doom compatible)
Dover Boys in Samsara
Everything's ridiculously tiny
The Vanilla Doom Levels now have invisible teleporters and subtly changed level part clones that make you go "That's not how I remember the level...?"
Daytime Drama's Troll-gun as a BFG replacement and is upgradeable in order of blood color
Bubsy's Revenge ("This could finally be my second good game!")
Instead of fighting demons, you have to debate the fuck out of them in a Tumblr/Reddit-like webisite
Play as Ralsei and put the entirety of hell to sleep (The pacifying hell) instead of killing it all
DK64 Weaponset for Doom
Flippy/Flipqy in Samsara
Doomguy's main and only weapon is the Star Rod
Mod where it's pretty much a compete exaggeration of what the anti-BD crowd thinks Brutal Doom is
You play as Flowey, and you make all the monsters infight each other instead of directly attacking them
H-Doom addon for Mr. Friendly
Some arthouse-esqe mod where all the textures are strange checkerboard patterns instead of sane texture design
Another arthouse-esque mod where you spawn in a huge, complex map.... and you can't do anything except look around.
A mod in vein of the first one except instead of checkerboards, everything's extremely abstract art to the point where you don't even know what's a wall, decorate, monster, or particle.
Yet another pretentious mod where you spawn in a sprawling and complex map... and you can't move in the x or z-axis.
Map where you have to walk a ridiculously thin, bendy, and long line to get to the goal, and it chucks a Rev in front of you near the end just to be an asshole
Doom but with clunky tank controls
Level where all you have to do is walk down a hallway to the exit... and it's utterly stupidly long.
Megawad with ridiulously complex rooms... and no keys, monsters, and pickups whatsoever.
Level where you can't see shit
Level that abuses the "You were hallucinating the whole time" cliche to no end.