Angel of Legacy
The Multiverse is full of marines who've fought Hell. They had it easy. Your mission is nearly impossible. Fortunately, their victories are your strength.
The game is significantly harder, you can't advance without a 100% killrate and all special levels are mandatory, but most badges and some medals provide bonuses or make special levels easier. Most badges and medals cannot be earned on this mode, but completion may have its own reward.
Ideas for bonuses: Feel free to suggest more
Cyberdemon's Head-Cyberdemon(s) is 10% slower
UAC Star (bronze/silver/gold cluster)-+5%/10%/15% dodge rate
Gambler's Shield-Levers have more beneficial effects
Untouchable Badge-+1 armor
Experience Medal/Cross-+10%/20% experience gain
Malicious Knives Cross-+15% attack speed in melee
Hell Arena Key-2 large health globes randomly placed in Hell's Arena
Hell Armorer Badge-Shambler has -2 armor
Technician Badges-More mods spawned
Armorer Badges-More specials spawned
Strongman Badges-+2 damage and +1 accuracy in melee per badge
Speedrunner Badges-+5% move speed per badge
Berserker Badges-Start with knife/chainsaw/Butcher's Cleaver/double chainsaw/ripper
Marksman Badges-Start with blaster/storm bolter pistol/Trigun/Anti-Freak Jackal/Grammaton Cleric Beretta/Trigun
Shottyman Badges-Start with double shotgun/assault shotgun/plasma shotgun/super shotgun/Jackhammer
Lightfoot Badges-Ammo stack size increased by 10% per badge
Eagerness Badges-+10% medpack effect and envirosuit duration per badge
Runner Badges-+5% move speed per badge
Inquisitor Badges-+10% powerup effect/duration per badge
Quartermaster Badges-The nuke level effect comes with +1 minute per badge
Brick Badges-Makes the wall thinner while widening the treasure room (for safer phasing) and making its wall thicker (to protect the loot)
Lava badges-Adds envirosuit packs to the starting island
Reaper Badges-Reduce enemy revive rate (global)
Longinus Badges-Adds berserk packs
This will make badges and medals more fun to earn.
Just to clarify how difficult this challenge should be to justify having all these bonuses, it should be nearly impossible on HNTR without any bonuses. On N! with every bonus, it should still be decently challenging.
Extra difficulty stuff: Also suggest stuff
Enemies reviving on any difficulty level.
Enemies taking less damage from acid/lava.
Enemies having increased sight range, damage, accuracy, health, armor, speed.
The Wall rebuilding itself.
The Spider's Lair webs-pull-to-center thing I've heard suggested.
The Halls of Carnage having a second slower lava wave that comes from the right.
The Arena Master trying to fix the fights by randomly blasting you.
Random health loss from the Angel of Death's aura of unlife.
Other stuff:
Option to disable individual bonuses.
Record of bonuses at end of AoL mortem.
Ideally, this mode would be a sort of ultra-campaign through which one accesses the "TRUE ULTIMATE FINAL SECRET BONUS ENDING" after getting really good at the game and deliberately going for every badge they can be bothered to get.