First, all Starbases need a unique function that makes them more than a big turret.
Suggestions for Starbase functions:
1) Planet Protection (TEC - planetary shield upgrade in addition to being an all-in-one type of space fortress ::: Advent - Mind Trick / Mass Reverie type of ability putting all ships within a bombardment range from the planet to sleep excluding siege platforms from the Vulkoras)
2) Phase Jump Disruption (Vasari - ability that disrupts all initiated phase jumps and shuts down all phase jump engines within range for 30 seconds - with a 30 second cooldown, stacks nicely with PJI's.)
The Starbase abilites should be AM consuming so that by disrupting their AM their abilities could be countered. This would give Light frigates a new function rendering starbases ineffective (until LF's are wiped out by the base).
Starbases should cost a combined amount of Tactical Slots, Logistic Slots and possibly Fleet Supply. There should be some consideration where to build them instead of spamming them into every grav well.
Starbase constructor ships should be built in the Capital Ship Factory instead of Frigate Factory. Cap ship factories could really do with more use. And the constructors should be a lot bigger anyway considering the enormous structures they build.
Upgrades for Starbases - If you research Trade Ports or Broadcast Centers you should be able to upgrade your starbases with one without having to research them again for Starbases. Having to research a similar tech twice is annoying and feels like a waste of resources.
It would be best if Starbases started out as cheap hulls that would have to be slowly upgraded. This would give a smart opponent time to destroy them before they grow in power too much and would bring another tactical aspect into the game. Casually building a huge battlestation into an enemy grav well before they can send in a fleet that is two jumps away is a little silly. The upgrades should be smaller and cheaper to keep the scale of a Starbase's power level reasonable. Especially with the hull/shield upgrades that seem over the top.
