Half-Life 2 took that further, and grounded a more sophisticated (but still familiar) story of near-future resistance in a believable dystopia. It has been widely imitated, and for good reason. It was a compellingly presented world populated by well-performed characters. Again, you felt like you were in a real place to an extent that you hadn't necessarily been in previous games.
In addition to advancing the FPS as a whole, then, Half-Life 3 would presumably need to advance our understanding of what a gameworld can be. This is where it gets much more complicated, I suspect. The advance of game engine technology has slowed. We're not blown away by see-saw physics any more. I wouldn't be surprised if Valve had experimented with VR with this in mind. Because what else could they do?