Games

You might be interested in Marc Laidlaw's (the story author for Half-Life) Epistle 3, the rough outline of the story plan for Half-Life 3 that he turned into a letter from Gordon and released after he left Valve. He has since said that he regrets releasing it, as game stories are wildly impacted by the design and development of the game itself, so the end result would likely be far different. Especially with Alyx retconning some things. That said, it's still an interesting look at what was planned at least at one point. Fans are working on a fangame based on the concept.

Likewise, there's the book Half-Life: Raising the Bar, which documents a TON of stuff about the creation of Half-Life and Half-Life 2, with a bunch of info on concept art and cut ideas. The original plan for the game would have been even darker than what we got and didn't feature Gordon at all! Unfortunately you'll probably just need to download a pirate scan of it, as I don't believe they've printed new copies in over a decade.

Together with everything the fans have figured out about the early leaked version of Half-Life 2 that matches up with the cut plot in Raising the Bar, you can build your own rich "fan canon" until Valve releases anything more.

I do think it was a missed opportunity to not have Laidlaw write some books set in the universe when they didn't need him working on game plot though.