Fashion.
Every leap of 50 years has to look completely different from the last.
Although, fashion in he Empire should remain static for the entire 500 year span of the story because it's soul is dead?
Or should their rates of fashion be cycling 4 times as fast because of the decadence without morality and an obsession of form over function?
Hmmm?
Is it still going to be about preserving atomic power, or are they going to invent a super science like dark matter engines or quark forges? Because 50 years into the Encyclopaedia the four empires had already lost the specialism to use atomic energy.
Coal powered spaceships?
No, silly. They had the old tech, ships that were 80 years old, the Periphery just couldn't build new reactors or figure out how to look after what they had very well.
But as far as fashion goes, it seems like they need to get all the losers from a couple years of America's next top Designer and put them in a sweat shop.