How about adding cages to the game, like some of the Final Doom maps had?
Possible ideas would be:
- Enemies could still attack you if you are in their range. Demons for example would be able to maul you if moved near of their cages.
- Risk that when caged monster is attacked the attack would break the cage and free the monster.
- Similary change that the monster would manage to break itself free by hiting it's cage hard enough.
- Have cages contain items instead of monsters (in the mock up the cage on the left contains green armor).
- Risk that using a lever would open all cages in a map.
- Risk that using a lever would drop a cage on the marine and you would have to break it to escape.
- Have them pushable (if they're empty) and have them block movement, but not the projectiles so they could be used to block corridors.