Suggestion: Allow us to embed arbitrary images (probably .bmp or .png) in maps, along with layering information (So we can say "Draw this over/under planets/ships"). Additionally, optional density - I guess we'll have to specify size as well. Might look something like this:
[entity]
icon="sun.png"
layer = top
density = true
xsize = 200
ysize = 200
You could also give no icon to just have an 'invisible barrier' sort of entity - could be useful when trying to match actual density with a nonrectangular image.