[Swarm]

Technological degradation

...The idea being that an empire has to spend a certain amount on research just to keep still. Below this amount and it will lose build, then repair capability.

I'm thinking of any number of SF worlds e.g. Asimov's Foundataion where an empire is fallen from its great days.

So an empire may have ships and technologies lying around from the old days which it can no longer build, or even repair.

This could lead to a number of interesting scenarios -- you might have Uber war vessels more advanced than anyone else -- but you daren't commit them to a battle where they could get damaged.

The research points required to stand still could be a simple function applied to the empire population (on the basis that large societies tend to become decadent and collapse), and age (on the basis that old societies tend to become decadent and collapse). These could be tuned to disable the effect entirely if desired. (Personally, I'm in favour of anything which acts as a brake on the ridiculous tech escalation in the game :-))