Personally I like to have access to fecal sludge because it lets me make plenty of chemfuel. At the same time, latrines reduce cleanliness and need emptying somewhat frequently. Sure you can use a septic tank, but that steadily cleans waste too so you lose out on the poop.

I'd like to see a cistern, basically a septic tank without the sludge removal. Something to unlock at the same time as the sewage outlet (Since it's just a sewage outlet attached to a tank).