The idea is to give the starbase a researched ability, say about the 6-8th lab range, that locks down the gravity well. By this I mean ALL ships being unable leave: enemy, allied, trade and the owners combat ships. Activation would break trade lines, thus carrying a hefty cost in larger maps, especially if done at stars. But what this would do is force the enemy fleet to engage the starbase, and give time for a defensive fleet to arrive. As a further balance, it could be given a cool down time after deactivation, making turning it off and on at a whim not quite so easy.
 This would open up the possiblity of owning and fortifying multiple systems without the aid of allies, making 5 star map domination by one player feasible.