Looking at my Azuran Sysem map and its conspicuous shape, an idea occurred to me: the AD&D "Great Wheel Cosmology" adapted to a science fiction setting. I've sort of toyed with that idea before in a science fantasy multiverse kind of way, but now I'm thinking more classic space opera in the vein of the things that inspired Traveller, but a bit more Vancian--as is appropriate to its AD&D roots.
So here's the pitch: Sometime in the future, an early spacefaring humanity encounters a gate and gains access to a system of FTL via hyperspace (or the astralspace) and gets its introduction to an ancient, galactic civilization with arcane rules and customs a bit like Brin's Uplift universe. At the "center" of the gates is Hub, a place with a gigantic neutral territory station--like Babylon 5 on a grander scale. Hub connects to all the various worlds. Here's a short sampling:
Archeron: A war world, possibly one where a decadent civilization has kidnapped warriors form different times and worlds to battles for their entertainment.
Beastworld: A planet where many animal species share a group intelligence.
Carceri: An environmentally hostile ancient prison planet.
Limbo: A world in an area of reality warping "broken space" where hyperspace spills in leading to a graveyard of ships.
Mechanus: Robotic beings out to bring order to the galaxy via assimilation. A somewhat (maybe) more reasonable Borg.
Pandemonium: A world only inhabitable in subterranean caverns, but even those are swept by winds that generate infrasound that can drive humanoids insane like the titular Winds of Gath.