I have been doing some thinking whilst commuting and somehow came up with the following rough draft for a setting/idea:
So in the distant past some extraordinarily wealthy individuals set off into distance space to colonize, expending all of their vast wealth in doing so and very much establishing this colony with the intention of setting themselves up as Space Nobles. They did this, and had the resources to set themselves up with an extremely well planned, high technology and rapidly developing base, only for all contact with Earth to be cut off without warning. They were deliberately not connected to the wormhole network used for FTL travel but were still alarmed by this and cautiously expand back toward the core.
At this point they find out that aliens have successfully conquered the human race and start a war with them, a war they are winning and have been for centuries, 'liberating' occupied human populations or isolated and technologically regressed worlds then welcoming them into new lives of being serfs for their genetically augmented and super tech possessing new overlords. The aliens have their own wormhole network but destroy it as they lose territory (Unless daring heroes thwart this perhaps!) meaning that the advance is limited to the speed of light, with new wormholes being brought forward to consolidate gains.
The game is set in a region on the edge of a cluster, with stars perhaps half a lightyear from each other (go for say 3x time in game, fighting across space to new systems is a thing albeit slow) where an advance force of especially ambitious/covetous nobles pushed far ahead, only to fight a bitter battle in interstellar space where their fleet was scattered and their wormhole 'back home' destroyed. They won and seized the sector capital, they are advancing and setting up a local wormhole network, they can still talk to everyone else via Ansible or whatever, but they have no way back and no way for massive reinforcements to arrive for say... 20 years.
There would however be a constant stream of scattered ships arriving from interstellar space, often unexpectedly, to allow new PCs to turn up.
Everyone would be trying to 1) Fight a war, 2) Set up themselves and their families to get as much as possible and 3) Try to ensure that when contact with the Empire is reestablished they are not executed for being Space Traitors but rather get to become the new Space Viceroy. Space Nobles are super capable, products of generations of genetic enhancement, and have really good technology, but cannot make new Space Noble grade ships locally (though starships can probably manufacture new smaller, expendable on adventures grade ships, Space Armour, Space Swords, etc). The aliens are dangerous but on the back foot, the 'Liberated' humans definitely better off under Space Nobles rather than Evil Aliens but likely remembering Space Democracy as a thing.
I do like the idea of the Aliens being Space Elves or something, that opens a lot more options for interaction than if they are bug monsters or robots or similar.
Originally I was thinking that all Space Nobles would have their own starship, albeit varying in size and capabilities, making 'ship owners' a 'class' works though. Reading the above I did think that you could have a divide between Conquistador type Space Nobles, and Space Navy Officers (who would be nobles themselves of course). The former would have more economic clout, likely mandates to administer worlds, ships which make much better Space Palaces and also manufacturies or able to land as a colony, etc.
Space Navy types would have more outright murderous warships (Though certainly not unique access to them), possibly more unity of purpose, etc, but would be cut off from their supplies and with no choice but to work with/for the more privately motivated Space Nobles, even outside of the fact that they have major personal incentives to do so.
