I know the game's not in active development, but I still think crafting theoretical additions to the game is fun. So here's two challenge modes I came up with:
Angel of Opportunism
"They say if you seek, nature shall provide. And what you seek is more guns! Every time you reload a weapon it becomes weaker. Oh, and don't bother with melee, infinite-ammo guns or the ammochain skill, unless you LIKE dealing 1 damage."
Reloading a shotgun, pistol or miscellaneous weapon would lower damage or sides by 1, whichever's higher (like an inverse power mod). Reloading any RPG or burst weapon does this twice (so rocket launcher becomes 5d5, then 4d4). Reloading a BFG does it 3 times since the numbers start so high. This continues until they become 1d1.
To avoid cheating: Weapons with regenerating ammo do this whenever the regen starts. Ammochain causes affected weapons to weaken once every time you shoot, as do infinite-ammo guns. Melee attacks all do 1 damage no matter what.
The idea is that you have to constantly replace your guns with new ones, mostly those dropped by enemies. It would still be an "easy" angel though. The higher tier badges could include 100% kills, conqueror, and combining it with angel of light travel. For balance purposes it should probably guarantee Mortuary and Halls of Carnage, since they include lots of human enemies while their counterparts have none.
Angel of Sloth
"For those who love to play cautiously. All player actions take 2.5 times as long. To compensate, you get +4 accuracy."
Archangel of Sloth
"2.5 times as slow isn't enough for you? How about four times as slow? Here's a protip: Don't miss."
"Very hard" and "blade" level challenges, or maybe even another tier up from that. Any mistake will be punished severely, dodging is useless, and you won't be able to reload fast enough to shotgun cornershoot. I'm not sure how you would even beat this except maybe racing for the Shottyhead trait. So obviously the highest tier badge will ask you to do it without finesse and/or as a Marine (also giving up the technician's quick item use).