I have little experience with actual game development but I’ve got a few solid hours of mapping in Source under my belt. One of my favorite features in the Portal 2 engine was the ability to create “worldportals” (not to be confused with the portals your gun shoots in the game).
It basically allows you to stitch together two parts of a map that are completely apart from each other and with some clever level design, could even make for some pretty mind-boggling rooms.
I’ve seen it done on the game Antichamber which was built in Unreal so I know its possible, just not sure how its done.