I'm afraid I'm going to have to be one of the 'party poopers'.
As much as I enjoyed the Half-Life games (though I agree that the first one was better than the sequel, primarily because the second game felt like a glorified tech demo for the Steam engine a lot of the time), I have zero confidence in any prospect of there being another game in the series, at least one that is anything like the first two (or four, I guess).
Why? Because multiplayer games have become Valve's bread and butter. Well, okay, Steam is actually Valve's bread and butter. But not only do multiplayer games make more money for them, they also increase the exposure of users to the Steam service more than single player ones do. The additional monetisation (which Valve has become a major fan of) they can employ with them and the fact that multiplayer games highlight fewer of Steam's downsides as a service (after all, people aren't going to care about online DRM with a game that's all about playing online) only serve as icings on the cake.
Simply put, multiplayer games have been a better investment for Valve. Under such circumstances, it just doesn't make much sense for them to make another grand single player-focused title, particularly in an era where heavily scripted campaigns have begun to fall by the wayside. So even if a new Half-Life does come, it will be nothing like the games that we knew.
Obviously, the old adage of 'never say never' applies. But given that it's been almost a decade since the last entry and that many key members of the game's development team have since left the company, I think it's far more likely that there is just no interest inside Valve to make another game in the series. And honestly, one could argue that this kind of result was inevitable since Half-Life 2's release; perhaps even sooner, since the popularity of the original Counter-Strike overshadowed the original Half-Life.
Post edited November 28, 2016 by Gandos