That you can put in your map!
Feel free to use any and all ideas, no credit required. If you have an idea, post here. Do NOT post anything else but ideas. Do NOT shoot these ideas down or discuss anything else. Just an idea or ideas. That's IT.
1) Crumble doorways- If you shoot/use projectile alien weapons, then some doorways would fall and be reduced to rubble that blocks the path. Skulks would be and to slip through small holes in the rubble with some quick manueverinng. Anyone in the path of the falling rubble would be crushed.
2) Exploding pipes- If you shoot/use projectile weapons enough on a pipe that contains something, it bursts and it's contents spill into the air. These contents could be deadly to marines, or aliens, or both. Some suggestions would be gasses that poison one race or both, fire, gasoline...