how about the story of Half-Life in another person's view. One that has no Idea what Black Mesa is, but is there during the alien attack? Ye know, like a normal dude transformed into an action hero, like Freeman, only he has nothing to do with Black Mesa. His house is destroyed, his family and friends killed, and now he wants revenge on the alien. He goes up into one of their ship (they have ships now) And goes on a quest to kill them (Meeting G-man on the way for some wierd reson.
OR! The Whole thing from G-man's point of view! An FBI man is taken in by the evil aliens, and goes out to fight rebelious aliens and his own government who is trying to stop the attack. He says that the aliens liked him and let him live.
JUst a thought.