City's on earth planets ??

yes put roads and ruins

Ruin objects acting like ready-built walls would be a nice obstacle to see on the map.

Think this is a really kool and unique idea, never saw this thread before so looking at all this and the idea's for the first time and I like it. I think you have to be carefull not to have too much on the planets though, otherwise it'll just look like a cluttered mess and loose the feel of the actual planets themselves, but having some sort of ancient civilization ruins or something would add a kool look to the planet environment.

l will try to make this a mod for release..
So far i have a greybox biome working in the system editor but it has some issues..
Some building's and trees sink very far into the ground last time i tried it in build v.57xxx.

I currently have the city biome appear on earth planets only when the temp is between 45-55 on the system editors temp slider,
It only appears in the middle of large area's of jungle, if there isn't enough space for the biome it wont appear.

I'm also working on a modified "desert Biome", the desert biome is used on desert planets and on earth planets too.
i'm going to add the occasional pyramid to the biome if the temp is between 50-60 on the temp slider in the system editor.

This idea.
I love it.
It would definitely add a really cool layer of atmosphere to the game.
It'd also be pretty cool to see some sort of signs/graffiti on walls that are saying things that are against the commanders/factions.
Assuming that the commanders were invented by humans.
Although graffiti probably wouldn't last (DATA EXPUNGED) years, but hey, in a game with self-replicating robots fighting by using giant rocket engines to guide asteroids into planets, I can suspend my disbelief a little.

That is fking sweet dude!!! man you should post some screenshots of what you've been able to do so far, but this is one of the better idea for a mod I've come across, that and the creation of totally new planet types/planet biomes all together.... keep at it and some of the buildings getting buried deep into the terrain might actually be alright to give it that much more ruin effect anyway as that is what would happen in real life anyway as the nature of environment takes over.