I have a quite weird idea, but anyways: I thought someone talented could make some kind of mod which would add a face hugger (from Alien, Alien vs. Predator, you know what I'm talking about) to the evolution list of the Aliens. It could somehow lay eggs in humans, and the "impregnated" human could become a one time use spawn point. If someone gave more thought to this it could make a really good mod.
Face Hugger:
http://www.zbrushcentral.com/zbc/attachment.php?attachmentid=12061