Here are some isolated suggestions, but of course in general all the pieces should be designed to work together -
More than one kind of slavery. Not in the sense of different pop-rushing mechanisms, but different civics that are named as being variants of slavery. Helots, for one.
Whatever the Serfdom civic is, +75% or +100% worker productivity. People don't seem to use it much at +50%; I don't understand why it's still in Vanilla (maybe I just don't know how to use it...?). It might be a good one to enable earlier, maybe as a builder alternative to Council of Elders.
Some kind of Democracy thing.
Depending on what is done with religions, options including God King (or call it "Pharoah" for flavor), Organized Religion (reduced maintenance?). Somehow reflect how religion was intimately tied into politics (at least in Rome, and I think Greece, and I think everywhere.) Keep theocracy but maybe give unhappiness for persecution.
Drafting: pop requirement = 4, pop reduction = 2, unhappiness = 0. That might break it... but try it. As is, it's not strong enough.
Migration available at Pastoralism. Also Pastoralism enables Goat Pasture (easy option for graphic: empty pasture), buildable on hills for +1 food. Migration: build units with food; +1 Food from Pasture and Goat Pasture; -1 Food from Farms and Fishing Boats; -50% distance maintenance.