This idea is heavily starcraft inspired, but I'm going to suggest it anyway (flame away !).
It would be cool to have a lifeform (skulk or a new one) that allows to take control of a killed marine (special attack by this lifeform), allowing the alien to virtually "become" the marine (with maybe a few tweaks, pro and con). This would allow a long range combatant vs the marines, with the ability to pick up weapons, but no way of using armories or the like so its basically a scavenger. Could allow the alien to drop the marine body when its useless.
Flame away !