My idea is along the lines of the movie Alien. You essentially seed the target planet with hostile xenomorphs which ravage it into a 0 level planet. This would be useful for destroying important enemy planets which you don't plan on defending or dont have the ability to defend against the Ai taking it back. I'm not sure whether to make it a sort of invasion mechanic like the spore modules the Korath had or to make it a timer. Each turn that passes by the xenomorph numbers grow and unless the invaded planet brings reinforcements by docking ships in orbit, the xenomorphs will eventually consume and render it useless. Each turn that passes by requires more reinforcement ships to stem the alien tide, essentially causing the enemy to divert his military to save the planets. A parallel for Good civilizations could be something like reverse engineered Peacekeepers. In an almost rise of the machines kind of way, these artificial Peacekeepers grow in number over time and require the enemy civilization to station reinforcement ships around orbit as well, however if thru rise of the machines succeeds after a number of turns, all planetary resistance is crushed and the peacekeepers completely isolate the planet from the larger empire, essentially turning it into 0 PQ but in a less horrific way since the people are alive and well albeit subjects of the reverse engineered peacekeepers.
I just wanted more flavorful ways to take over planets and this idea strikes me as flavorful but not OP. Thoughts?
PS I love your work devs! I make stories of galactic drama every game in my head lol