Whether or not we see another Half-Life game after the excellent Half-Life: Alyx is entirely down to the capricious whims of Valve, but one thing we can be sure of is that it won't be in the form of Half-Life 2: Episode Three. For starters, simply too much time has passed for Valve to make another straightforward expansion to Half-Life 2. But there's also the fact Half-Life 2's writer, Marc Laidlaw, released his vision for the story of that elusive final episode.
That said, we might, just might, get to see Laidlaw's vision for Half-Life's concluding chapter in videogame form. Enter Infinite Finality, a new Half-Life 2 mod that is based upon the "Epistle 3" plot outline that Laidlaw released back in 2017.
For those who don't know, Laidlaw's plot summary for Epistle 3 centred around the Borealis, the mysterious cargo ship glimpsed through a TV screen toward the end of Half-Life 2. According to Epistle 3, the next chapter of Half-Life 2's story would have seen Gordon and Alyx travelling to the Borealis, where due to the ship's connection with Black Mesa's rival corporation Aperture Science, some strange interdimensional shenanigans would have occurred.
The project announced its existence back in April, and work appears to be continuing apace. Early images of the mod show rugged, icy maps laced with the Combine's distinctive architecture, alongside some stranger-looking environments indicative of the Borealis' more surrealist effects. They've also created new skins for the Combine and zombies to better suit the arctic setting, and even given Alyx some more weather-appropriate attire, which is considerate.
It's worth noting that this isn't the first time a bunch of enthusiasts have tried to finish Valve's game for them. Project Borealis, an even more ambitious attempt to recreate Episode 3 from Laidlaw's outline, started up not long after Laidlaw originally posted the plot summary online. The Borealis team has been very quiet over the last couple of years, but progress is apparently still ongoing, with the team looking to fill "short-term positions" back in 2022.
The Infinite Finality devs specifically reference Borealis in their FAQ, stating, "we won't try to be as ambitious as Project Borealis is" and saying "everything will be released" even in the event that the project doesn't see completion. The team's also currently on the hunt for programmers.