Well, it only took thirteen years, but it now appears likely that a proper sequel to Half-Life 2 and its expansions is finally on the way.
MAJOR, MAJOR SPOILERS FOR HALF-LIFE: ALYX FOLLOW
The news came in the closing moments of the latest game in the Half-Life series, Half-Life: Alyx. An "interquel" set between Half-Life and Half-Life 2, the game was expected to be a VR showcase and a stand-alone story that would not impact on the future of the series. However, the game's ending is a huge surprise and ties directly into the cliffhanger ending that we were left on thirteen years ago in Half-Life 2: Episode Two.
The game follows Alyx as she gets wind of an artifact that the Combine has hidden in City 17's Quarantine Zone, an area where the flora and fauna of the alien border world of Xen has manifested on Earth. Early in the game it appears to be a weapon of mass destruction, but as the story progresses Alyx's supporting team, including her father Eli and "guy in the van" Russell, discover that it is in fact a prison constraining an individual in temporal stasis. Combine data files confirm that the individual escaped from the Black Mesa Research Facility and raised hell along the way. Eli concludes that the prisoner is none other than Gordon Freeman (the playable character in Half-Life, Half-Life 2, and the latter's two expansions).
After numerous challenges, Alyx breaks into the prison and opens it, only to find it isn't Gordon at all but instead the mysterious G-Man, a familiar figure from previous games in the series. The G-Man tells Alyx that he is grateful for her intervention and offers her a reward, a "nudge" in time. She asks him to make it so the Combine never invaded Earth, but he refuses, saying that would be far more than a nudge. Instead he offers to change the future for her. He shows her the moment five years in the future that her father, Eli, is murdered by a Combine advisor in the closing seconds of Half-Life 2: Episode 2, and offers her a chance to intervene. She does so, killing the advisor before it can kill her father. The G-Man then cautions Alyx that she is now his new operative, to replace Gordon Freeman whose performance is becoming "unsatisfactory." Like Gordon and Adrian Shephard (from Half-Life: Opposing Force) before her, Alyx is recruited into the G-Man's private group of operatives and seemingly removed from space/time.
There is then a flash of light and, post-credits sequence, the player resumes control of Gordon Freeman at the very end of Episode Two. A confused Gordon sees a now-alive Eli in front of him, the Combine Advisor dead in the background, and Alyx nowhere to be seen as Eli rants about this being the "unforeseen consequences" they were warned about previously. Eli vows to kill the G-Man - who can briefly be seen walking off in the distance - and throws Gordon's crowbar to him, telling him he has work to do.
The ending retcons that of Episode Two, in which Eli is killed and we end with Alyx weeping over her father's body. Writer Marc Laidlaw later revealed how he planned to resolve the cliffhanger in Episode Three, with Alyx and Gordon journeying to the Aperture Science vessel Borealis which was caught in multiple time frames and using it as a bomb to sever the link between the Combine homeworld (revealed to be an enormous Dyson Sphere) and Earth. At the last minute the G-Man would appear and rescue Alyx, but leave Gordon to his fate. Gordon would be saved and returned to Earth by the vortigaunts, with Alyx now missing and Gordon planning to find and rescue her, possibly going up against the G-Man directly.
It appears that Half-Life: Alyx has fulfilled some of the same story criteria whilst changing things around, so that Alyx's recruitment takes place at the end of Episode Two (presumably her memory of the intervening period was suppressed or eliminated) instead and Gordon's search for Alyx and a way of defeating the Combine once and for all will now simultaneously play out in a prospective Half-Life 3.
And does this mean Half-Life 3 is on the way? It appears so. Valve know their customers and wouldn't have an ending like this unless it was setup work for a future game in the series. It's also clear that Alyx was partially a way of updating their tech, engine and assets in preparation for an even more ambitious project, with the pre-release interviews for the game leaning heavily on the idea that more Half-Life content will be coming down the pipe (global pandemic notwithstanding, of course).
When Half-Life 3 might appear is another question, but fans may take heart from Alyx's unusually fast development time (the game was developed in about three years, surprisingly efficient for a modern AAA shooter). The real question is in which format it will appear. Half-Life: Alyx got away with being a VR game because of its status as a side-project. Making Half-Life 3 itself VR-only would be vastly more contentious and controversial, but if the sales for Alyx are good enough maybe Valve will be tempted to go down that route.
In the meantime, fans and theorists are going to have a field day working out what this all means for the slow-gestating franchise.