I was thinking about ogame and stellaris for a while when i came up with the idea of have defensive buildings palced in tiles of the planet, like the shield generator it would need to be operated to work and it wouls deal periodically damage to a orbiting enemy fleet. In this moment planets are pretty unprotected when they lose their space station (that happens very often) and only another fleet can defend them from the siege. It can be a resercheable technology and behave like the ship weapons: ground flack artillery, missile launchers, Gauss Cannons, Plasma Cannons, Laser turrets, Ion cannons and stuff like that; so the repeteable tech could be applied to the planetary defenses. You could make bastions and protect border planets from the first strikes of a war or make Synergy with the resilent trait.