Great game and keep up the good work!
I have some suggestions in order to improve gameplay, planning, tactics, co-op and choice of playstyle:

Make some diversity with abilites mobs has.
Some mobs (Maybe Slaanesh daemons) dodge left and right which will make slow projectile weapons useless (Plasma).
Some mobs (Maybe Khornate daemons) increase damage and speed when hit which will make low damage but fast weapon dangerous to use. (Hitting them several times with low damage will make them enraged, use few but heavy hits).
Some mobs (Maybe Chaos hounds) have a "lock down prey" ability, which means if they hit their ability, you have to take it out first because you can't move because they are locking you down.
Some mobs (Psykers, Sorcerers) can somewhat tamper with your mind, which means you can't stand still, you just keep running or you just keep on firing your gun and can't stop. (Some sort of "failed" mind control against the mighy inquisitor) This leads to that you must control WHERE you are running while in combat or WHERE you are firing.

Make improved AI with different roles. (healer, Tanks, DD(Damage dealer), CC(Crowd Control)).
An enemy healer will run for cover and heal.
An enemy tank will stand in the way, soak damage and force you back. (Maybe have a shield, which forces you to go around?)
An enemy DD will take cover if possible and deal damage or charge once close enough from the back of the tank to you.
An enemy CC will try to force you to move around a lot or just to pin you down in one spot.

Some of the above can be displayed in the information window before to start the mission E.G:
"Scouts report that they have seen Khornate Daemons in the area" or
"Ordo Hereticus reports that a Psyker might be in the area"

As a professional concept developer I am happy to help out. Just PM me.

Kind regards/
Aerifax