Thanks to everyone for replying.
My current idea for game/supplement:
At some point there is a "Galactic Empire" spanning thousands of worlds/stars.
The Emperor is considered "divine".
You have to be a relative of his to have any real power - so each ruler of a planet/solar system is somehow related to him.
The closer to him blood-wise, the closer you are system wise. So, the guy/gal running Earth for example is a distant cousin, 4 times removed, etc.
Every 4 years the Emperor has a party and it is a great honor to be invited. Some planets always have representatives at the party, others (maybe like Earth) never get any invites. Even rarer at these parties does the Emperor personally honor/promote someone.
There is one system that is "forbidden" - at some point a relative of the Emperor ticked him off and they wiped the planet out in retribution. Over time, rumors and myths about this planet emerge.
Ships jump from system to system using "jump points".
There is a rebellion on one system, the rebel leader somehow sabotages the jump point - knowing how the Imperial space navy would arrive (thousands of ships jumping at once to make a great impression) - his sabotage of the jump point destroys most of the Imperial navy and the empire begins to crumble. There was so much energy released that this basically became a "nova".
Now the other part of the "story" is a fantasy world which is set in this "universe" - occasionally the adventures may come across a futuristic item (an item from the old empire). So there would be "elves" and "dwarves" and such (and maybe new things), and rarely some item that doesn't "fit" in this world.
The GM could set his story at any point in the Empire (pre-empire, pre-tech, empire, post-empire).
Maybe one of the players has "divine powers" (meaning they are somehow descended from the old Emperor). Or maybe the entire race of Elves is from this line.
Maybe "runes" are the language of the old Empire.
A space adventure could be investigating the planet the Emperor destroyed or maybe the ruins of the ships that the jump point destroyed.
That's all I have so far. I'm hoping that my brother (who writes small-press comics on the side will help me flesh this out.) No surprise to anyone I'm sure, but being married and now having kids pretty much takes all your time (in a good way) and it's hard to work on this stuff anymore.