I've said before, to give us both a larger variety of basic Civ types and cover historically important and interesting groups that the game has not modeled well in the past, I think the game needs two things:
1. A decent model for the City State polities that at least makes them competitive - and given the modern examples of 'city-states' like Singapore and Dubai there's no reason they can't actually strive for certain types of Victory Conditions right to End of Game.
2. A model for the pastoral nomadic groups that dominated central Asia for almost 2500 years and had a huge impact on their Civ-normal settled neighbors.
. . .
I can think of Nomadic civs somehow similar to Oceanic civs, that in my view, should be able to 'float' cities on water, and move them (Beyond Earth mechanics) but
on land. Maybe envisioning a lighter city version, with limited growth possibilities. A sort of 'mobile' encampments-barracks cities, so relevant on the eastern side of the Urals,
and the Buffalo people of the Great Plains.
These kind of Nomadic civs unique traits parhaps should rely on an elastic environment, and without such a seasonal - environmental changes, it looks hard to enact
some of these unique civs traits that could surface - I was thinking of ''subterranean'' rivers for desert civs, adapted to roaming hordes of horses, or buffalos leaving some sort of
traces on the terrains, that only nomadic civs could see and thus exploit... naturally (a certain tech could reveal these food-resources spots to other civs at one point), before
addomestication kicks in.. Tundra civs should be able to see these as well. I think of Russia, Finns, specifically, other than Mongols, Turks, American Indians.