I'd like Sub-Surface Facilities and Structures to be added to the game so players can create unique content based on real world locations.
This would mean having an entrance at surface level that lead via tunnel into the underground facility or cave system, maybe a man hole with a ladder leading down into a small entrance point which would then lead into the facility.
Or, a cave system that has an entrance the player can just wander into. This type of addition would make the game really fun to mod and play.