I think a good way to start a DM style map is to start by just drawing out the flow / paths that you think would be a fun place to run and jump around in. Usually after sketching for a little while an image will pop into my head about a real world space that might make sense for the map.
Something else to consider is that the map doesn't have to "make sense". I will gladly give up realism for a fun map. Shoot, you could make an all gray Lego block space map with some deranged smiley face for a sun orbiting around the play area and if the gameplay and flow are solid I'll play it.
If you have never heard of a geocomp before its worth checking out if you are thinking of mapping:
These are just a few examples and most of these maps use no more than 5 or 6 textures but they are cool to run around and frag in.