Right now the game just has two that I know of, increased hordes and reduced hordes. While this is a great idea in concept it doesn’t really work that well as-is.
Firstly, with the reduced hordes, it’s mostly fine as it is as it affords a player the opportunity to ‘stress-test’ their skills and gear at a higher difficulty, but it should also come with fewer resources available during a mission to compensate. The heretics have already been through, looted most of the map, and left behind a reduced force. Ammo pickups are rarer, or the large ammo sacks are reduced to ammo tins, medical stations have fewer charges, that sort of thing.
With the larger hordes, the combination of both higher numbers of trash and higher numbers of specials can be overwhelming. Instead of just “increases all enemies”, it could be split into three separate moifiers:
Endless Horde: The cultists are trying to bury you in bodies. Fewer special enemies, but you’ll be scouring the map for ammo an health because of how many cultists are going to throw themselves at you.
Elite Traitors: Fewer enemies, but a much higher proportion of flak and carapace armoured enemies. Shotgunners, Snipers, Ogryn, Flamers, etc.
Deep Corruption: Higher proportion of chaos-corrupted enemies. Lots of Poxwalkers instead of cultists and melee soldiers. Hounds, Mutants, Beasts of Nurgle, Plague Ogryn, etc. are more common than other specials.
Each of these three has a nice theme and encourages a different playstyle and weapon choice from each other.