[JonathonW]

That'd pretty much be Portal; the technique he's using to render there is basically the same as the one Portal uses (the Portal engine can do "invisible" portals like this too, but there's only one point across both games where they're used in the finished game).

I suppose something around non-Euclidean levels without user-created portals could be interesting, but I think it'd be hard to flesh that out to a point where there's enough there for a whole game without coming up with some other gimmick.

[wizzwizz4]

I know it was left as an exercise for the reader, but I never worked out where that was used. (The turret live fire exercise comes to mind, but I thought I'd ruled that out.) Do you happen to know?

[wizzwizz4]

Remembered the entity name, and this information is on the wiki page: https://developer.valvesoftware.com/wiki/Linked_portal_door

> They are also used twice in the campaign (contrary to the commentary's claim that they are only used once): in the GLaDOS wakeup sequence where they are used to connect the incinerator shaft to GLaDOS' chamber, and in Finale 2 where they are used to connect the "trap" chamber to the main map. These are the only uses of this entity in the final game.