Right now, all I know is that the official founding of the Empire was an even similar to the economic and political ties that the Hapsburgs made with all the rest of Europe during the 13th-15th centuries.
As for the Big Fifteen, here's what I have so far. The basic political structure is similar to the relationship between early Industrial cities and surrounding rural areas/townships.
1. Earth: Now focuses on administration, tourism and historic preservation for the human race. Much of what passes for aristocracy in the Empire come from Earth.
2. Caan: A world where tree species never evolved and where there are no true mountains, making grass the dominant plant form. Caan and it's colonies are the major breadbasket for the Empire.
3. Zion: Mountainous world settled by the descendants of Hebrews, Lebanese and Palestinians. The society of Zionist colonies has evolved to be matriarchal, and they are the site of a corporate mining powerhouse.
4. Hemeldaal: An extremely hot and humid jungle planet settled by the descendants of Afrikaaners and other Germanic groups. As one might expect, the source of it's status as a superpower comes from its wealth of botanical products.
5. Winter: An ice planet that thrives on commercial animal genetic engineering and breeding programs. It and its colonies are the main source for animal goods.
6. Atlantis: It's a cliched name for a waterworld, I know. But I honestly think that's what we'd name a waterworld space colony if we had one.
7. Jah: Lies at the heart of the Imperial trade network and is the sight of the Empire's intergalactic stock exchange. Think if Dagobah were settled by Ferengis.
8. Jigoku: The intergalactic silicon valley. Jigoku is all urban, since the inhabitants live in the vast subterranean chambers that pock mark the interior of a world who's only native life is still in the earliest stages. Haven't figured out exactly why they went through all the trouble of settling here, yet.
9. Inferno: Gotta have a fire world, too! Heavy volcanism, lots of industrial forges, Imperial shipyards, engineering projects. Tons of geothermal energy for sale.
10. Ul-Suhra: Another very flat world where most of the water is locked in ice caps, leaving what were once seas as vast salt pans. The planet's location makes it the top intergalactic space port, with almost all major routes from one end of the Empire to the other having to transfer through here.
11. Mamangu: The Hollywood planet of the Empire, where all the important entertainment production is done and where most of the major entertainment and art celebrities live.
12. Mundus Novus: Settled by the descendants of Martian colonists. It serves as the top major religious destination for pilgrims. Haven't figured out why yet. I also haven't decided how humanity's religions have changed in the future. I do know that most of the major faiths die out and are replaced.
13. Meru: Meru is to the Empire what China is to our Earth - when you think of where the average manufactured goods you buy are made, that's the kind of place Meru is in the game world. Only this is a low population density planet where most of the grunt work is done by robots and simple AI, and its population trace their ancestry to South Asia.
14. Gaia: The home base for the largest shipping company of the Empire, located at Alpha Centari.
15. Royava: The home base for the Empire's equivalent of Amazon/eBay, Facebook and Google all in one. Most commerce in the Empire is conducted digitally through the intergalactic telecom network, so these guys are sitting as pretty as cyber-sultans.
Incidently, there's no aliens in the game, though the players will sometimes have to courier or 'liberate' alien technology that was discovered by the Empire's sanctioned exploration vessels. Player 'races' are all human sub-races that have diverged on the various colonies.