Half-Life 2's third DLC episode was never finished because Gabe Newell and company didn't just want to finish the Half-Life 2 story arc, but to do something big and innovative with the game design, and as Newell put it in a new documentary, he was "stumped."
But also in that documentary, released for Half-Life 2's 20th anniversary, there's footage of some pretty cool-looking Episode 3 stuff, including an ice gun that could be used to build transparent barriers, but also in "kind of like a Silver Surfer mode" that lets you run across ice ramps as you build them. The detail that there'd be an ice gun was known previously, but as far as I'm aware, this footage is seeing the light of day for the first time.
