This is definitely looking like one of the more promising and in-depth expansion style mods, I can't wait to use it.
Beyond Earth gives me a few ideas for how a significant change in gameplay might occur - namely, something analogous to the alignments, but based more in governmental style. Basically the idea is something like the Prehistoric Era mod where the tech tree is pushed back to say, Fire, but cities as they are don't exist from the onset, rather you end up playing slightly more like barbarians with mobile encampents as your base. Taking the tech path towards agriculture and later writing encourages settling and ending up along a Sedentary alignment, more militaristic and pastorialist techs encourage a Nomadic path, and a middle ground centered on Archery and Trapping is...Tribal, I guess. These eventually roughly align with Monarchy, Horde, and Republic, and eventually, Order, Autocracy, and Freedom, but they're not mutually exclusive and indeed, it'd work a lot like Purity, Supremacy, and Harmony in ways.
All this would probably require heavier changes than is possible though, and it wouldn't play nice with a lot of things. But yeah things like that, more active and evolving city-states or even more shifting between befitting statuses - something like Victoria 2 where the major powers would be full civs, secondary powers would play like city-states, those that are fallen behind would be uncivs that play like barbarians, but things can shift between those states as the game progresses.
at this point though that's pie in the sky stuff that's practically making a Civ 6 by hand. So I'm mostly just spewing ideas of what might be neat.