Is it possible to incorporate a happiness system that effects how you can collect taxes? Maybe also effecting your production and research.
For example lets say you have taxes at 50% and only 60% of your population is happy.
Now you have some sort of resource that makes your population happy and you collect 60% taxes and your population is still 60% happy.
Better yet, the happiness of your planets should effect your warships and planets by allowing them to turn over to the enemy influcence or into ordinary barbarians/pirates.