Well they released two expansions for Half Life. They said they will have an expansion already in the works. It would be probably be an rebel fighter or something.
What would be really cool in the expansion, if they did a game before Gordon shows up and tell you the main story on how the Combine actually take over the world. I would find that really interesting.
Blue Shift and Opposing Forces.
Never played Blue Shift, which added high defintion characters and was played from Barney's perspective.
Opposing Forces which was the best expansion pack of that year, was played behind a commando left behind. The game rocks. It added a couple of new weapons, a barnacle grapple beam, a sniper rifle and also was a second perspective to Gordon's viewpoint. Example. The part where Gordon jumps in the teleport for his trip to Zen, you actually see him jumping.
Blue shift was pretty stupid, at least with opposing force there were new enemies, weapons and I thought it was very clever when you visit places that gordon goes. In blue shift, the basis of the gameplay was mostly precision jumping (Which sux).
If they do make an expansion to HL2, it better have new weapons and enemies.