I remember when playing MOO2 that there was a planet called Orion. It was always hard to get to, but once you colonize it you received new unique weapons and ship. What if GC3 had a planets (let's say 3) that once you colonize them this would increase your offense and defense greatly. But if you loose even one of them the bonuses will no longer work. Or at least concept similar to this one. 
Cheers!