I have an idea for realism: "Diplomacy".
There are many parts of it hidden in the coding, the most noticeable is political control. Look out for "political influence" or "political control". Each planet's alignment means something, if it's Dark, its imperial friendly, and if it's light, it's rebel friendly. Each unit has an affect to the happiness, the more units u have garrisoned, the happier the population will be. If the population is too mad, they will revolt and the planet will join the rival faction. I know that there is a video somewhere out there that shows it in-game but its in german.