I see POPs, Ethnicity and Nomad mechanic (usefull for early game) as part of the same system. Like this:
> POP are the basic core mechanic of the game, they have identity parameters like Ethnicity, Religion and Class (farmer, warrior, etc.)
> All main and minor civs start the game with a TRIBE unit that can settle and turn into a VILLAGE (camp), but these villages can also migrate and turn back to tribes. In others words an one tile unit/infrastructure that have some city-like basic functions. These tribe/village can have a small number of POPs and when settled can even growth to produce more tribe/villages, armies and develop techs.
> Playable civs can growth villages to cities at Ancient Era (or even better you need to growth a city to reach Ancient Era!).
> Minor (non-playable) civs would either want to also turn on city states as fast as possible (like Elamite or Olmec), others would be mainly settled as villages (like Celt or Miao) while others would be nomads moving a lot in their tribe form (like Scythian or Comanche).
> Minor tribes/villages would give you Barbarian Clan like interactions, for example hire mercenaries.
> Once subjugated (by force or diplomacy) you can take direct actions to turn their villages on new cities or accept their POPs inside your previous cities. Each ethnicity is linked to bonus, unique units, techs and resources.