Despite Half-Life 2: Episode Three being the most infamous vaporware ever (not yet) made, Valve was also planning another game in the series, often referred to as Half-Life 2: Episode Four, between 2007 and 2008, despite the game being intended as stand-alone. The tie-in game, more commonly called Return to Ravenholm, was being produced by Arkane Studios in collaboration with Valve and was to take place between Half-Life 2: Episode One and Episode Two. The game was cancelled by Valve due to the creative constraints of making the game stand-alone yet fit into existing continuity.
Back in 2012, Valve designer Marc Laidlaw told fansite Lambda Generation that:
“[T]here was indeed a project called Return to Ravenholm. We are big fans of Arkane and wanted to come up with a project we could work on together. We threw ideas around, they built some cool stuff, but we eventually decided that it didn’t make sense to pursue it at the time. As I recall, we felt like a lot of the staples of Ravenholm–headcrabs and zombies!–were pretty much played out, and the fact that it would have to take place sometime before the end of Episode 2 (so as not to advance beyond where Valve had pushed the story) was a creative constraint that would hamper the project…and Arkane.”
The cancelled title was thought lost to the ether, but Robert Wilinski, Senior Environment Artist at Arcane during development of the game, has released some screenshots, via ValveTime. A sad glimpse at what could have been: