Thank you for the great mod! I've been playing it infrequently since it's conception and find myself coming back to it every few years.
I have a suggestion regarding the discussed possibility for nomadic civilizations.
I think it would be best to keep the amount of new features to a minimum, that is why I would suggest adding only one new type of settlement, the Nomadic Camp.
The Nomadic Camp would function like a city in most ways except two: it doesn't exert cultural influence (so no cultural borders) and it doesn't work individual tiles, instead it would gain the combined yields of all the plain tiles in its area.
As such it would only benefit from grazeland (so plain steppe, grassland etc.), and also from camp and pasture resources (game and cattle), and oases. Improvements would not benefit it, which would encourage pillaging.
Nomadic Camp should be mobile in the sense it can be relocated (turning it to a nomadic settler unit), but moving it should mostly be necessary when securing better grazing grounds or retreating from invaders.
The only way to get a Nomadic Camp should be training a settler unit from another Nomadic Camp or spawning with one. If the location of a Nomadic Camp is covered by cultural influence, it would need to relocate (or turn to a normal city if it is the owner's culture).
The Nomadic Camp should have its own (limited) roster of "buildings", conceptually themed to be "nomadic" or "tribal" (eg. Horse Breeder: increased yields from horse resources, free Mobility promotion for new horse units). Population size should influence how many "buildings" one Nomadic Camp can sustain.
In terms of production or science the Nomadic Camp should not be able to compete with well-developed cities. It should however allow for nomads to thrive in areas with few resources but lots of land for grazing (eg. Central Asia, Arabia). Nomadic Camps should be able to produce high quality archers and horseunits quite efficiently but lose their competitive edge towards the end of the game period. Their habitats should be organically slowly occupied by sedentary populations.
Therefore there wouldn't be any inherent difference between a sedentary and a nomadic civilization. Some civilizations (the Turks, Mongols, Arabs, also barbarians and independents) would spawn with Nomadic Camps and acquire regular cities through conquest. Then they would rule both sedentary and nomadic populations side by side until possibly loosing their nomadic settlements alltogether (either voluntarily or to foreign conquest) and become fully sedentary.
Disregarding my suggestions, I am interested if there are indeed plans to feature nomads in some form or the other in the upcoming bigger map. Thank you!