|Korhal, the capital of the Terran Dominion in Starcraft|
One thing that I've been rummaging through my head is how I want the Commonwealth of Free Planets to feel like. I definitely want to keep a frontier feel about it, but at the same time, I do want points of human and alien civilization so that I can have interesting urban and political adventures. For this I've found a great deal of inspiration with Starcraft, the implied setting for D&D 4e, and the TTRPG Clement Sector.
Both are settings that take place out in a frontier area and have their fair share of worlds to explore and colonies to meet with. But they also have that 'points of light' style of campaign where you can have large capital worlds that the players can run through for fun. So using Starcraft's Terran Confederacy/Dominion as an example, for every Korhal capital planet out there, you'll have plenty of Mar Sara backwaters out there for 'lawless adventuring'. Which to me, is the core of exploration. Lawlessness and lack of social rules and etiquette so the players can (and many times have to) use their wits to overcome difficult obstacles. While the points of civilization are mostly safe havens until trouble happens (or they go looking for it). Then they have to use the same wit and canny but with the limitations of the social center they are in. Which to me, can bring about the most innovation and ingenuity from players. Conflict + limitations = ingenuity.
So the Commonwealth have been there for centuries. With a decent enough population growth, I see most of the Core Worlds with a population in the tens to hundreds of millions. Most would be centered around the first colony of the planet, with many of the later ones acting as satellite cities. You'd probably have the occasional colony far away for reasons unknown (resources, religious, or simply tired of the main colony's shit). Leaves plenty of undiscovered wilderness on the planets for intrepid players to explore, while still having cities for other adventures. I'm thinking five main core worlds would be good enough, with the oldest being the most populated and probably the capital. Soon I'll have to give these planets names and maps, but for now, it's a bit up in the air.