I think it would be interesting if after you conquer ~50 planets you can then start to harvest blue crystals which allow for higher tier upgrades and then at ~75 planets you can get green crystals which allow for top tier upgrades and if you collect enough of both and conquer ~100 planets you can add an extra slot for upgrades to your weapons, so instead of 3 different upgrades you can then have 4, and once all are cleared you can have 5.