Uhm, related to the sanitation/water building discussion, I always felt sewers were kind of a big deal, and it would work nicely with health and plague. In my opinion they played a big and universal role in human development and were a real show of grandeur for empires overall, even in secluded America they were developed with no contact to the old world.
Now I don't know if others share the same experience but I do have a hard time keeping my cities healthy for long periods of time, and having to switch trade routes to other cities to prevent plague can be annoying when the others barely make money. Since CP adds those food stealing events I felt population count raised its value to me even more.
I feel a sewer early game building would be pretty nice, no requirements but with just a really high maintenance (maybe scaling per era? probably thinking to much about it now). That way these will mostly just end up in your biggest cities in the pre-modern age just as IRL, which allows you to keep your traderoutes in the most rewarding cities. You can sell them when things get economically rough (medieval kind of rough!) and place them in all your cities in later eras when your income is ridiculous.
Also it would be nice as an early alternative to draining when you are stuck in swampland, and I reckon won't conflict with Tomatekh's Harappa that replaces the aquaduct, it could just be called the sewer system/sanitary sewer.
Regarding the purpose of slaves, I myself did actually end up buying them mostly in colonies! But slave plantations are pretty old (latafundia), and so are colonies, so early game slaves could still have the same purpose right?
Maybe getting too into it now but if peasant need to stay prominent for the player give them a 'corvée' or 'serfdom' upgrade that lets you expend them on 1 tile for an increased tile yield for x turns. Would be a nice alternative to having them on standby. Or you could just like, restrict slave purchase to only colonies