Let's do this deductively then; All but two districts in the game get its first building in the same tech that unlocks the district itself. Those two districts are the Aqueduct and the Neighborhood. It's also important to note that as of the build we have the most comprehensive look into - it would seem that the Aqueduct/Neighborhood only get
one building between them. I personally can't identify anything that looks like an additional building for either district that doesn't at least look as if it belongs somewhere else.
Sanitation and Urbanization fall, era wise, in the exact same spot in their respective trees - the end of the industrial era. So either; The Sewer is an upgrade to the Aqueduct that comes several eras later. Would make sense.
Or, The Sewer is the first building of the neighborhood and they unlock at relatively the same time provided progression balance. This is also makes sense, given the pattern of the rest of the districts.
You could say that Sewers being an Aqueduct upgrade is more likely because that gives two separate mid-late game housing boosts. One tied to tech (Sewers) and one tied to Culture (housing). Plus, past civ games typically have a progression from aqueducts to sewers.
Or, you could note that the last 3 major governments have themes and require specific buildings and therefore districts to be built to boost them. Communism requiring factories (industrial zones), Fascism requiring Military Academies (Encampments), and Democracy requiring sewers (In question).
Though Democracy's bonuses don't really fit a theme tied to a district, however the next question I would ask would be, what's more likely - having four aqueducts in your empire or having four neighborhoods.
I'd be inclined to think Neighborhoods may be more common.
The other thing to consider is that there can be multiple Neighborhoods, and so any buildings built in Neighborhoods could potentially have multiple copies. Not sure that makes sense for Sewers.
Even under the multiple neighborhood model, the difference between the tiny housing area we've seen and the apartment complex models could be something as simple as an upgrade. Such that if you had multiple Neighborhoods. You'd only need to construct one "sewer"
All of that said, Sewers extend everywhere in the city. So despite my wordy post I still think it's entirely possible that Sewers could be yet another building constructed in the city center meant to increase housing for the city. I mean in reality, you'd have sewers beneath
all of your districts. This possibility keeps the idea of having two different mid-late game housing increases coming from each tree - one from tech (sewers), one from the civics (neighborhoods). Oh and also, this would mean that neighborhood and aqueducts get no buildings, which at least retains some level of consistency rather than having 1 district get it's first building 3 eras later, or the other district get it's first building at roughly the same time but from a different research tree - all while the rest of them unlock their first building at the same time as the district itself. It may be more likely that aqueduct and neighborhoods simply have no buildings.
Personally, of the three options - Aqueduct is lowest on the totem pole for me.
