Anyways to chip in my ideas for planet settling:
When you place a colony, over time there should be some urban sprawl spreading from it, like some smaller domes, greying of the surrounding landscape and city light effects to represent growth and any hills/mountains or uneven terrain nearby becoming flat or terraced to accommodate urbanization while canals are dug in some areas. The larger sprawls would have skyscrapers and arc blocks popping up which, besides being nice eye candy would make for some fun to drive rovers down the streets or fly between them.
It would be something in tiers, like first tier it's a outpost with a limited set of building options, then after so much resource collection you can upgrade to a settlement which is a domed facility with built in resource refining and buildings and unlocking new building options, next level is a town, then city. If the air on the planet is breathable then the dome is removed from the settlement.
Another thing for colonizing a planet I think your race should have an AI that will naturally start spreading across the planet and setting up new settlements once you get the first colony self sufficient. It would make things a lot easier instead of having to place every single city on a planet by hand. To control the AI though there should be a panel allowing you to set planetary colonial policy with options such as:
- Quarantine: For when you find a planet with unicellular life but it turns out said life comes in the form of a disease that kills off two thirds of your race. The planet is forbidden and none of your race will visit it.
- Forbidden: Garden planet. No one is allowed to colonize the planet and if you put some defensive satellites in orbit they will prevent other space-fairing races from colonizing it too.
- Restricted: Only you can place colonies and the colonist will only build within a short range of it while using as little of the planet's resources as possible.
- Moderate: The AI will spread towns and cities across the planet, but will stop after a certain point while being mindful of natural resources. Areas around cities will probably be stripped of resources over time and there might be some minor negative effects on the seas and atmosphere if you're not paying attention to the planet and building structures to counter this. This would be the closest to Earth's level of industrialization.
- Unrestricted: The AI will cover the planet in cities and towns, no resources on the planet are off limits, the atmosphere and sea will turn toxic quickly if left unwatched and if there's a biosphere it will suffer if not go completely extinct as will the colonists eventually if you do nothing to keep the environment stable. Terraforming facilities and other artificial means will be needed to keep the atmosphere and water clean. If you can keep the planet stable it will eventually turn into a city planet after thousands of years. Planets with no atmosphere might develop one of smoggy sulfuric compounds if the conditions are right.
Another aspect of colony building is unrest. Things that could cause unrest would be lack of resources, cosmic events like an asteroid impact, war, or mass immigration from another planet (which could be caused by the previous things). If the unrest gets too bad then the planet will fall into civil war and you can watch little low poly tanks, aircraft, and spacecraft destroy each other as the planet's population steadily drops and if the situation doesn't solve itself and you do nothing to intervene then the war will get more violent and result in orbital colonies and asteroids around the planet getting damaged, their orbits altered and potentially de-orbiting either from damage or intentionally and nukes, chemical weapons, and nanobot swarms being launched on cities, while damaging the atmosphere, eventually destroying all life on the planet and turn the planet into a toxic, cratered hellhole.
A civil war in an escalated state could raise unrest on nearby planets/moons/asteroids, cause a chain reaction, spread to other star systems and cause a lot of "FUN".
This comes to the final idea: Ruins.
Ruins happen when the people abandon their planet and the cities on them, or die off en-mass. The ruins will stick around for 5,000-10,000 years. Anyone that happens upon them can explore them with a rover and find out information about the former colonists, like what happened to them, when did it happen, did they die, If they left the planet which direction did they go, and what other planets were colonized by the race at the time of abandonment? Eventually though after so many thousands of years the ruins will break down and nature will take back over, leaving only the leveled terrain, canals and terraces behind, which themselves will eventually erode completely after a million years. Far into the future all that would be left are some craters from possibly large former colonies that deorbited and trace amounts of anomalous gasses in the atmosphere. Though this all depends on the body the colony is on too. An airless body would keep any land changes as there is no environmental erosion and the ruins might last longer there.