Given that we have four elements that exist in progression, I found myself doing that with the nations themselves. As for nomadism being the most ancient, you can argue that, but my thinking was that someday we'll move beyond the excesses of our modern world with its nation states and onto something with more simplicity and mobility. That's why I put that ahead of the nationhood we see with Earth and Fire. The Air Nomads have always struck me as more enlightened, not more primitive.
"Primitive" is a loaded and condescending term, one best avoided in an anthropological discussion. There's no simplistic upward ladder in cultural development any more than there is in biological evolution. There's just adaptation to the demands of one's circumstances.
It used to be believed by Westerners -- and by urban-dwelling Asians -- that the horse nomads of Central Asia were more "primitive" than agrarian or city-based societies, but in fact they started out as agrarian peoples and later adopted nomadic pastoralism once the introduction of rideable horses gave them a new means to exploit an environment that had previously been unexploitable. Far from being a regression to an earlier state, it was an adaptation made possible by a societal innovation, the breeding of horses into riding animals. Similarly, it used to be believed by Westerners that Native Americans were more "primitive" than Europeans because they didn't use metals to the same extent or in the same ways, but now we understand that they'd simply developed in a different direction, one focused more on mastering organic materials than inorganic ones, and were actually much more advanced than Europeans when it came to agriculture, selective breeding, construction based on organic materials, etc. The natives' longbows actually had greater range and accuracy than the colonists' crude guns.
And of course cultural or philosophical enlightenment is a totally different matter from the sophistication of one's subsistence methods or technology. There's no simple upward progression there either. Technological or material innovation can promote enlightenment, for instance by making education more available or giving more opportunities to women or minorities, or it can work against enlightenment, for instance by increasing the demand for slave labor and exploited factory workers or by facilitating the spread of destructive propaganda.
That takes me to another issue… I'm surprised that they have territory. Given that they're nomads, I'd think that having temples among the other nations' territories would be sufficient and in fitting with their enlightened nomadic nature.
On the other hand, nomads inhabiting other cultures' territory often find themselves under pressure to assimilate. There's a long history of nomadic peoples being subject to oppression and persecution, or otherwise having difficulty maintaining their distinct identities. Ask the Roma. Or ask the Jews why they felt it necessary to have their own homeland.
Besides, nomadism doesn't necessarily mean having no territory at all. Lots of nomadic cultures engage in seasonal migrations between established sites, coming back to them year after year. The Air Nomads' migration among the four Air Temples seems to fit this pattern.
The Earth Kingdom seems more like early Europe while the Fire Nation seems more like the later British Empire. Similar, but one does feel a little more "advanced" than the other. You can see it if you squint.
Actually the Earth Kingdom is based on 19th-century China while the Fire Nation is based on Imperial Japan. Again, there's no simplistic hierarchy of "advancement," just different choices and adaptations. For most of the Common Era, China was the most advanced civilization on Earth. The reason the Industrial Revolution happened was because Europe was strongly motivated to improve its transportation methods in order to gain better access to the wealth of China, and to improve its manufacturing methods in hopes of competing with the wealth of China. When Europe then forced Japan to open its borders, Japan embraced industrialization, enabling it to race ahead of China as well. But they also embraced aggressive cultural imperialism and conquest, which can hardly be considered more enlightened.
So, again, there's no innate "superiority" to any of these cultures. A culture that's "less advanced" in one century may become "more advanced" in the next -- and the pendulum can swing back the other way in a later century. For the past few generations, the United States has been the most technologically advanced society, but now our educational system is under threat from growing cultural conservatism and we're rapidly losing ground to India, China, and other countries.
Neither of those characters looks any more "Caucasian" to me than any of the other Earth Kingdom characters in the show.