Perk-i

Just tie available cap ship crews directly to empire population instead of having the hard cap. If you start losing planet populations, you don't have crews to replace capital ship losses. This would give you more incentive to capture planets and also give you more capital ships to use to cover back doors. This also makes the populous worlds much more attractive targets; you can defend them in tight, and the enemy has to come into your defensive perimiter to make a dent.