I suggest packed management of map files: instead of sparse files in different folders, each map could be a ZIP file, containing all what is needed (like quake 3 arena & related). Example: Zip with "ARENA.pms, /sceneries, /textures, ..."
This will give these advantages:
- one file per map
- more clean installation
- maps can be deleted completely
- save disk space (gfx is usually in BPM format)
On loading, game should extract files in a temporary folder, if there are missing files search in the common paths (ex. /textures) else if critical elements are not found, close with error and download the package.