Whenever I get reading stuff on the HL wiki, it makes me think that the universe lends itself really well to a semi-open world game, along the lines of Arkham Asylum in that there are several large connected levels that expand as the player gains more technology and defeats various enemies.
For example, you're running around in a combine controlled city trying to avoid being detected while building up the resistance. You raid weapon and food caches, and develop bases of operations. The devastation and depression of the people could be highlighted, as Gordon has to make the tough decision of who to provide with resources. The player could build up little bases everywhere, or a few key bases in key locations that Gordon could retreat to when the S hits the F. Finding scientists and bringing them back to the well defended bases would give the player a source of weapon and item "research," providing access to more weapons and items like the gravity gun, or pheropods, also opening more of the map up.
With some ai tweaking, the combine would patrol for Gordon and crack down on the resistance. On discovery, raids would be called in, with headcrab shellings or combine swat invasions. Stalkers would patrol the streets, and would have to be avoided until the player had enough weaponry to confront them. The player could come back to a base he didn't adequately equip that is now entirely zombified. The combine could move in and out of an area based on control, ala GTA:SA's style gang wars. I imagine a turning-point mission where the player dismantles a combine conversion center and makes it into a main base of operations for the resistance. I also see the vorts being a pivotal part of winning the war for the city, with the crescendo being the destruction of the tower in the middle of the city. G-man could make several appearances and provide riddler-esque problems at various locations. I also imagine, not just because I'm using arkham asylum as a model, but because I think it fits in with the resistance theme, a photography mini-game where evidence of the combines treatment of individuals is documented and used to encourage more people join the resistance instead of complacently be controlled.
I could see excursions to xen being a fascinating addition, but I want to see where HL3 takes us, since that seems to be the logical next step after Ep3.
I realize that HL2's linear narrative essentially tried to give the effective feeling of this idea, but I really like the idea of controlling how I build up the resistance, and forcing tough storyline based decisions on Gordon. Also the idea of returning to a base that has been destroyed because you didn't provide them with enough firepower and were too close to the frontlines gives me a certain satisfaction that I want out of all the tower defense minigames that seem tagged onto triple-A titles.
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