I think a Civilization-esque game would be a wonderful direction for an Aliens game to go.
Maybe have a bit more in the way of a city builder, managing a colony or a hive, battling it out for dominance of a map.
Barring that, I wish they'd just bring Extinction to PC with a proper skirmish mode and online multiplayer. T_T
As for an Aliens FPS, atmosphere and story will be key. I liked the toughness of the Warriors in AVP2010, at least on the difficulty mode I played on. I don't think the Xenos should go down easily unless you have a really powerful gun. Forcing the player to think about whether facing off against the Xenos is a wise option would be nice.