Exo waves for every Civilian Leader!
Seriously though, I kind of dislike how they give effectively infinite AIP reduction right now, with the only bound being time you want to spend waiting.
There could be some kind of periodic small scale AI response which would be easily defeated on it's own but with 5 other civilians + normal waves + exo waves + w/e else works as an additional way to make yourself feel like you are stretched thin. Scale up the frequency (not strength!) of the attacks with the amount of AIP you have reduced. This way capturing and holding lots of civilians gives a good death by a thousand cuts feeling. IMO the best way to lose things in the game isn't to the mega waves of doom, its the slowly being eaten away because you just haven't planned your unlocks/systems/defenses well enough to stop the bleeding of things you want to protect.
Alternatively/additionally, have a cap on the amount of AIP reduction you can gain. If you capture a civilian leader before the first minute you can gain a max of 10 AIP reduction from it. If you capture it in the 10th hour you can get a maximum of 20 AIP reduction from it (offset by the +10 AIP it already generated). Perhaps have some cool consolation prize for reaching the limit, say the Civilians were scientists and their research provides an extra 500 knowledge each, or a neat cap ship to use (perhaps scale it so that the first civilians give mk1s, the last ones give mk5s?), or some other kind of flat benefit that doesn't incentivize doing nothing for 50 hours so you can win with AIP permanently floored.
On a sidenote, whose to say that having 8 Devourer golems roaming the galaxy wouldn't make for a hilarious game? Lets make this happen!