One thing that has bothered me in GalCiv2 has been how troops are recruited directly from your planet's population pool without as if they were ready made soldiers. Also, when your planet becomes invaded, you defend with your normal population of citizens. My idea is to start dividing a planetary population into a civilian population and a professional soldier population.
You could create a planetary building that converts civilians into soldiers. Then if your planet is attacked you could choose to defend with either trained soldiers or consripts (your untrained civilain population). Balance things by giving civilians a advantage penalty when attacking or defending to reflect their lack of training.
What I like about the idea is that it reflects the way most states train their militaries now a days while and also stops important planets from being depopulated during attacks. After all, when states conquer territory they go after military objectives first, then they tax the surviving civilian population (or you can enslave them if you're the Drengin ).