Like in Total War: Shogun 2. When you conquer star system or home system of some rival faction you could have option for vasalisation, because more systems you have the more overcolonization problem there is. That system then would provide you with half of all its resources. It would continue to develop and create fleets which would then aid you or rebel if your empire is in trobles like loosing war or low happines. Youd have new dialog options in diplomacy screen with that empire or system.