It's a couple hundred years in the future, Earth has gotten real shit, FTL travel remains categorically impossible. Corporations run a lot of stuff but not as much as you'd think. Specific-purpose AIs are everywhere but don't think like people, general AIs all have crippling mental illness and act like some combination of a drugfucked hippie and a scary neckbeard who beats you at an MTG tournament. No idea what system you'd use to embody this setting. We might come up with something.
Network of solar power stations orbiting Sun. Crawling convoy-towns that stay on dark side. Panels that stay fixed on planet's surface, are maintained by nomads during dark season, provide enormous amounts of power during bright season. Human-uninhabitable computer cities of energy-basking AI. Cancer rates through the roof. The horribly burnt guy from the movie Sunshine. Weird perihelion effects means sun goes up and then back sometimes, is great for shenanigans. Hideout for criminals who really don't want to be found. Solar sailors preparing for the long run to Jupiter, hauling a small fortune in charged batteries, hiring mercenaries to protect them from pirates. Capital is sapient knowledge-collecting wheeled city named Marduk.
Partially terraformed. Cloud cities. Reflective balloons that block sunlight. Upper atmosphere breathable, since oxygen compounds float on denser air. Half-built solar shade at L1 Lagrange point - Fresnel lens, long abandoned, inhabited by scavengers. Sulphur-eating bacteria. Hydrogen bombardment - produces graphite and water by Bosch reaction - hydrogen imported from Jupiter. Carbon dioxide frozen into dry ice, transported to Martian atmosphere. No space elevators - rotation too slow, geostationary orbit too high, plus storms. Capital floating mirror city of Tiamat, also sapient, adopted mother-goddess personality, complicated relationship with Marduk.
Home to 5/6 of human population. Space elevators, some collapsed. Partially terraformed - carbon-fixing bacteria designed for space use. Overheated - cyclones, blizzards, flooded cities, massive population displacement, slavery, plague, famine. Cities destroyed by orbital cannons, Lunar coilguns, engineered viruses, broken dams, fallen elevators, good old-fashioned nukes. Countries still exist. Tendency toward smaller political units rather than larger ones. Pacific Northwest earthquake triggered American civil war - independent Cascadia, etc. Secessionist South invaded Mexico. Cherokee state in Oklahoma. Pakistan-India atomic exchange brought on by flooding Bangladesh. China has state-loyalty high scores and cheevos and is unsettlingly stable. Saudi Arabia completely collapsed when bottom dropped out of oil market. Middle Eastern borders rewritten by war and Accord of Jerusalem. Paris poisoned by neutron bomb. Greenland, Antarctica, Siberia made habitable. Asteroid early-warning stations. Space junk. Major countries - Pangermany (EU successor state), Russia (kleptocracy, quasi-anarchist, decimated in war with remnants of NATO), Brazil (communist), Nigeria (cheerfully libertarian), the Confederated States of North America (capital Washington), the United States of America (capital New York), China (unchanged), Japan (zaibatsus lol), South African Alliance (China-backed, capital Harare), Neoborneo (pirate republic, 30% on fire at any given time), Iran (democratic, dominates central Asia, leads world in fallout-scrubbing technology). Independent Scotland, Catalonia, Punjab, Xinjiang. UNCE (United Nations and Corporate Entities) based out of Buenos Aires, hilariously ineffectual.
Crater colonies joined by underground railroads. Domes anchored to bedrock. Mass drivers - coilguns that launch spacecraft, decimate Earth cities. Militaristic culture, many naval bases. States that primarily exist off-planet (Taiwan, Free Mexico, CSNA-funded Israel, various Arabic royal families) often based here. Capital at Shackleton.
Partially terraformed. Greenhouse effect warms poles, sublimates dry ice into CO2, feeds greenhouse effect. Multiple parties competing to bring in gas - frozen ammonia from outer planets, methane from bacteria (not Titan - too far), synthesized sulfur hexafluoride and halocarbons. Phytoplankton. Cyanobacteria. Reduced albedo - algae, dark dust shipped from Phobos and Deimos. Shafts drilled to release core heat. Atmosphere, climate generally comparable to summit of Everest. Projects to induce magnetosphere currently on hold after several catastrophic failures. Underground cities of early settlers now considered historically interesting, have colonial re-enactors and associated genres of frontier fiction. Mons Olympus a transnational park and lightly trafficked tourist spot. Resurrected, miniaturized Ice Age species thrive in the frigid environment, grazing off lichen and altitude-adapted bamboo. Sealed canyon colonies running whole length of Vallis Marineris. Capital at Noctis Labyrinthus, though nobody uses the old names any more.
Way stations, trade routes between Mars and Jupiter. Mining for exotic materials and metals for zero-g spaceship construction - more cost-effective because not stuck in gravity well. Hollowed-out asteroids converted to ships and living space. Four types of space rock. C-type, water-heavy, more common past snow line (distance from Sun at which ice forms), used for agriculture. S-type, silica-heavy, mineable. M-type, metal heavy, rarer than S-type, even more mineable. B-type, rare enough to be mostly mystery. Currency is gold. Population density approaching zero. Persecuted religious groups, banjo-plucking hillbillies mutated by incest and improper radiation shielding, sheriffs and petty criminals, truck stops for Jovian convoys, pirates, raiders, bounty hunters, company towns, people who've gone weird from being alone. Capital at Kerwan on Ceres.
More than twice as far from Mars as Mars is from the Sun. Four Galilean moons - Io (volcanoes), Europa (chaos terrain, tidal plumes, cryovolcanoes, colonies in frozen chasms and around deep-sea geothermal vents), Ganymede (bigger than Mercury, layers of ocean sandwiched between by phases of ice), Callisto (underground lakes, farthest from Jupiter's radiation, crater-saturated). Other moons also inhabited, similar to Belt but more heavily populated. Dark sides preferred for radiation shielding. Cultures no longer correspond as closely to those of Earth. Renaissance-themed? Silk, brass, astrolabes, pyramids, decadence, noble houses. Science fiction gets softer - quasi-psychics, city-sized hypermedusoids engineered to eat radiation and survive Jovian atmosphere. Prehistoric fish, gigamoebas, tube worms in underground seas. Capital at Thebes - submerged sacred metropolis built into mid-ocean ice phase on Ganymede. Major ports at Lofn on Callisto, where impact created gateway to subterranean water deposits, and Narbeth Chaos on Europa.
Even further out than Jupiter. Couple of diehard loners scraping out a living in the rings.The Jovian houses have funded a tiny outpost by the shores of Jingpo Lacus, a hydrocarbon sea on Titan, and it's a carefully guarded secret what goes on there.
Unexplored. Probes launched with AI pilots, prefabbed colonies, embryo banks and child-rearing algorithms. None have returned. Theories abound about Plutonian superconductors, machine cults, post-singularity utopias in the Oort cloud.