Some day, maybe, in another life... we'll get Half-Life 3.
The reality of Valve's dev cycles right now means that despite Half-Life 2: Episode 2 ending on a cliffhanger back in 2007, the "slot" a third instalment would've filled has passed. The nearest thing fans got to closure was a story summary posted by an ex-Valve employee, detailing the way the plot would have gone, as assumedly the real project has been canned for good.
Also, with the majority of Half-Life's impact happening in the early 2000s, there's an emerging teenage demographic the industry will want to target that have no idea what Half-Life even was. The games' impact was never as world-shaking as something like Grand Theft Auto, and despite the very notion of a third instalment being talked about to the point of becoming myth, it doesn't make financial "sense" to ever put it out.