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 been 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.
It's reminiscent of Portal 2's gels. There were also some neat blob enemies that used Portal tech—if you want to see 'em, the bit of the documentary that covers Episode 3 starts at about an hour and 53 minutes in.
I've queued it up in the embedded player above and it's on YouTube here. I guess I must not be Valve dev material, because if they'd put out Episode 3 with the ice gun and blobs in 2010ish, I probably would've been fine with it.
Sure, it wouldn't have blown my mind like Half-Life 2's Striders and Gravity Gun did in 2004, but you know: ice gun, blobs, the rest of the story.