To be balanced, add a new lever effect with the identical message "The smell of a massacre..." that instagibs you instead of enemies. Do not include this change in the version notes.
From now on, playing on ITYTD move will automatically colour your marine pink. Instead of blood, you leave trails of flowers. Hits are replaced with 'distance hugs' and deaths with 'just remembered they were late for a meeting'. Guns are friendship beads (2d6 damage to meanness). The full ending is unlocked by showing the cyberdemon your Thermonuclear Bomb Protest Sign and accepting his invitation to tea with his best friend Jesus Christ.
Arachnotrons are wussy in melee, give them a touch of death attack that bypasses armor and invulnerability.
Killing the cyberdemon with a melee weapon splits it into two fully healed cyberdemons.
Barons of hell can now pick up and activate nukes.
Splash damage from explosions will now detonate rockets as well as barrels. This includes any in your inventory. The damage stacks.
Megaspheres are now ten times more common, but have a 90% chance of being a trap and spawning up to 8 arch-viles around you.
Vulnerability globes. They tire you out, reduce your health to 1, unload all of your weapons and dump your entire inventory and equipment on the floor. On the upside they can remove cursed armor, but they cease working when less than half of the level's enemies remain.
Paint mod pack. It doesn't increase any stats, but your flaming decal BFG has never looked so cool.
Jumbo med-pack. It heals you up to 200%, but you immediately vomit and are left helpless for ten game seconds. Consuming more than one increases all your speeds by .05 each use.
New unique: Antimatter Bomb. It destroys the entire planet, leaving you floating in space until your invulnerability wears off you and splatter. Partial victory, I guess?
New unique: Home-ing Phase Device. It resets your experience and sends you back to phobos base. It tires you, but isn't destroyed upon use. Unique hoarding time.
Special level: Hell's Lavatory. It's like the lava pits, only with acid and no enemies or rewards. It replaces Hell's Armory with a 50% chance.
Angel of 1000. Expect JC caves by level 150.
Angel of 1. For those with really short coffee breaks. Oh, and the one level is phobos arena.
Angel of Apollo: You begin with a (cursed) nano-modded rocket launcher, red armor and plasteel boots. You may swap armor, but if the set you are wearing is ever completely destroyed you instantly suffocate.
... wait a second, that last one sounds awesome.