I'll share some ideas I went through
Idea 1: Payload on Mars, starting in cave, then going out to surface in a glass tunnel.
Scrapped due to being unnecessarily large.
Idea 2: Technological tunnel with spinning metallical rings forming walls and ceiling.
Scrapped due to func_rotating not working correctly.
Idea 3: Stalagmite spammed cave, with crystal light shining inbetween the stalagmites.
Scrapped due to shadows being crap and hammer refusing to compile with map-wide lightmap scale of 2.
Idea 4: Underground teleport. Multi-part doors slowly unbolting, lights coming on, and teleport that activates and sends you into white nothingness.
Scrapped due to being more an activating sequence then a cool tunnel and Source not being able to handle 5 dynamic lights on one surface.
Idea 5: Gothic hallway.
Completed.