I don't really know the lore too well but there is a backrooms site with like 1000 levels that might help for ideas :)
A level resembling level 18, found here Level 18 - "Memories" - The Backrooms that can transition to level 19, found here Level 19 - "Attic Floorboards" - The Backrooms (Specifically revision 12) could be a fun 1,2 punch of an experience.