Personally I think that trying to design Nomadic civs is a fools errand, and not a great way to portray any historic group, even the ones you are calling "nomadic".
There are cultures that move around a lot, but pretty much all of them have a territory, or a set rotation of places that they move back and forth to, with a range of a few hundred kilometers at most. This is seen mostly with pastoralist cultures moving back and forth from winter to summer grazing lands, but even some agriculturalist groups like the Haudenosaunee moved their communities around in a sort of slash and burn crop rotation.
You are confusing pastoral nomadism with trans-humant cultures. Pastoral nomads follow their herds/flocks. They may follow them in a set pattern, but they are always willing (and frequently able) to change that pattern if climate/terrain/other factors make it necessary or desirable. And frequently the area they cover is so comparatively huge that the 'set pattern' is not apparent to outsiders - look at the descriptions of the Scythians, Lakotah Souix, Comanche, or Goths in Ukraine by their more settled contemporary neighbors.
Trans-humant cultures move at specific times between specific places, most commonly between winter and summer settlements to take advantage of optimum terrain and seasonal climate. Best examples I know of (and most familiar) were:
1. the Algonkian communities of New England, many of whom moved annually between a summer village on the coast where they could exploit fishing and shellfishing, and a winter village inland out of range of the winter storms off the North Atlantic.
2. The pre-urban communities of Mesopotamia, who moved between summer pastures and fields in the foothills and winter villages 'down by the river'. Once they started developing irrigation that made it possible to grow crops/food even in the desert-like summer conditions, they started staying permanently in proto-cities located on/near the rivers and floodplains. The Trans-humant culture is enduring, though: 5000 years later the Kurds in the same area still prefer to 'take a vacation' in the hills in the summer while living 'permanently' in the cities of the plains.
IF we choose to model both, they would almost have to be modeled a little differently.
Nomadism in certain cultures is usually a result of mass migration and displacement, which we have attributed to some groups and not others as an accident of historiography. This is more likely to happen to pastoralist cultures because the land they regularly occupy is more marginal, more vulnerable to climatic shifts, and their lifestyle isn't dependent on static geographic features like a river valley. The Goths are remembered as a nomadic group moving all across Europe and settling in Spain, but they were farmers driven from their lands by the Huns. They existed for centuries in Ukraine and for centuries in Spain, with farms, cities ctc in each place, but to the pop-historian they are defined by their time in the 5th and 6th centuries spent as nomads, because that was the most dynamic and memorable time for that culture. So, are the Goths a "Nomadic" civ?
As
@BuchiTaton has pointed out, being a pastoralist culture implying a lack of urban centers is a false dichotomy as well. Should there maybe be less emphasis on districts and specialists with a pastoralist civ? Maybe, sure. I don't think you accomplish an accurate portrayal of pastoralist civs by prohibiting them from founding cities or building districts, however. I think you portray this lifestyle best by inducing civs to play wide, to put more emphasis on tile improvements over districts, and on creating abilities that spread fewer
citizens over more territory. I wouldn't ban urbanization, but give certain carrots and sticks for pastoralists to spread their population over more land and more, smaller cities (wide), while you give agricultural/sedentarist civs carrots for playing with fewer, larger cities (tall).
Again, you appear to be conflating two different mechanisms for 'nomadism'. On the one hand, Pastoral Nomadism is a lifestyle choice by some cultures. Once you had access to wheeled vehicles and horseback riding, using those 'technologies' to mobilize your population to follow (and guide) domestic herds and flocks gave a far better return on time invested than sitting next to a river trying to grow crops, and made the group less vulnerable to vagarities of terrain and climate. Even absent the wheeled vehicles, the pastoral horse-riding American Natives of the great plains used travois/sledges to move all their worldly goods and families from place to place, and groups like the Lakotah, Comanche, and Kiowa abandoned 'conventional' settled (primitive) farming or hunter-gathering for the nomadic lifestyle because it was more efficient on a return on time invested.
On the other hand, groups of all kinds have been forced to move because of changes in climate/terrain (Climate Change, as I have said before, is NOT a new phenomena in human history, it has been a Constant in human history) or the aggressiveness of their human neighbors.
This started early, since there is copious archeological evidence of cities being abandoned or cultures moving their 'core territories' from long before history started (i.e., long before writing systems) and kept right on happening. Just yesterday I read a report on DNA analysis of Post-Roman Britain graves that discovered a relatively massive migration of population from northern mainland Europe (tentatively, Northern Germany/Belgium to Scandinavia) to parts of England in the 5th century BE (immediate 'Post-Roman'). This was not, from the archeological evidence, a military-like invasion, it was a resettlement of entire populations and villages across the channel - a moment of 'nomadism' in which a population, for reasons not yet entirely understood, packed up and moved wholesale to a 'better world' while retaining most of their cultural and technological practices and knowledge.
I don't think any separate mechanisms are needed in-game for the second type of 'nomadism' - if a city becomes unviable for whatever reason, create a Settler and 'move' it.
The difference, at least in the Ancient to Renaissance Eras (or, roughly, the first half of the game), between the Pastoral and Settled cultures, though, has to be shown in-game if the game is going to continue to bill itself as a 'historical 4x' game.
The pastoral cultures had too much influence on their neighbors, in technology (among other things, the spoke-wheeled chariot, virtually all horse-controlling technologies, composite bow, possibly lost-wax metal casting, all started in pastoral cultures), in trade contacts and all their consequences. Silk Roads, in a word, but also the technological and cultural contacts that flowed along them: cast iron working, some gunpowder technologies from China to Europe, along with less desired elements like Bubonic Plague and less predictable elements like Christian, Islamic and Buddhist religions.
Any Pastoral Group that stayed around for any length of time either conquered or built their own 'cities', the latter probably starting as seasonal trading gatherings similar to the 'Rendesvous' of North America and becoming more permanent - like Batu Sarai of the Golden/Great Horde on the Volga. More commonly, 'pastoral' groups simply conquered and took over existing cities: Samarkand/Marakhanda was a major trading city for the Persian, Sogdian, and Kushan 'Civs', all pastoral at least when they are first identified.
) or any Civ with a map reliance. How do you balance versatility and historical accuracy?
modifier towards building settlers that scales for each 
