Anyone here play DayZ? How awesome would it be for a world like that to contain Aliens? (I think there's even been an Alien mod for one of the Arma games). Just imagine wandering around a huge landscape, somewhat sparsely populated save for other humans, either random or those who've entered in your party, and AI (or, f**k it, human controlled) Xenomorphs wandering the landscape.
It will never happen, but I can dream.