There should be a stability bubble kind of system.. as in every parsec outside of a bubble a system is, tax production will be reduced by 5% and thus bubble can be created by an "Imperial starbase" or something that has to be constructed (only one per system). This starbase can create a bubble of say a 30 parsec radius and the defensive bonus could be for example 1 defense per 10 population on the planet that builds it. Starbases should also have a credit upkeep to stop players simply building them everywhere- In order to be profitable the starbase would have to be build in high population and regions of the star map quite far from other bases.
This can will bring some more balance to the game I reckon by reducing the efficiency of larger empires as would happen in reality- like the Roman empire- and give smaller players a better chance against larger invaders.
Another idea is a rebellion system. every system outside of the bubble can increase the chance of planetary revolt by 1% each day with a limit at 50% if a revolt occurs, one random planet outside of the bubble should revert to native control with say 100 fighters spawning too. This will make the game much more interesting for larger players and slow the expansion of empires as they get larger- ultimately balancing the game more