Cultures, Pops and Tags
In 304 BC, there was a strong, shared Chinese identity (in those times called Hua 华 or Huaxia 华夏; the modern term Han 汉 hadn't been coined yet) that nonetheless had significant regional differences in spoken dialect, material culture, music and the like. These regional differences are mostly known through archaeology or anecdotes in written sources, so the boundaries are not known in great detail and there is a great deal of room for interpretation and informed guesswork. In addition to the unambiguously "Chinese" populace, there were also non-Chinese living in state societies (like Yiqu, Zhongshan and the former kingdom of Yue), in agrarian tribes (like the various Man and Baiyue peoples), or as nomads (like the Qiang, Xiongnu or Donghu), across a broad range of ethnolinguistic groupings (Culture Groups). That is to say, the in-game Warring States would be populated not just by "civilized" Chinese pops, but also non-Chinese civilized and uncivilized ones, and the mix of these different cultures, culture groups and pop types will be important for modeling (and balancing) the various Warring States. Here, I'll do a region-by-region breakdown of my suggestions for the demographics of each area, as well as the tags that I believe should be present there.
Nomenclature is tricky here, and it may take some creativity to create a good, descriptive name for each needed culture. There's also plenty of room for debate on my conclusions here, so I invite feedback and criticism from other posters on this.
I wrote and posted this from my phone, but I'll be drawing maps to accompany this and will edit them in when I'm able to.
North China
This area, centered on the Yellow and Wei rivers, is the center of mass for the Chinese culture group and to six of the seven major states, plus a number of smaller ones including Lu, Song and Zhongshan. The Chinese cultures of the area are, in my opinion, best divided into Western (Qin or Guanzhong, centered on the Wei River Valley), Central (Zhongguo or Guandong, in the middle reaches of the Yellow River), Northern (Jin or Sanjin, in the upland states of Wei, Zhao and Hann), and Eastern (difficult to name, but centered on Qi, Yan and Lu; could maybe be split in two).
In addition to the Chinese majority, most of the states of North China also had a non-Chinese minority; Chinese sources do a poor job of describing them in detail but we are still able to do a rough division. In Qin (excluding their lands in Sichuan, which I'll describe in the South China portion) the main minority are Rong people. In the Three Jin region, the Di are the primary minority, but in Zhongshan they made up a particularly large part of the population. Yan starts with small numbers of Di, but historically conquered large territories populated by Donghu and Gojoseon people in the game's early decades. Qi would have its own small, well-integrated Yi or Dongyi population.
Only the area immediately around the Yellow River would have a purely Chinese population, but many of the Rong, Di and Yi pops should be slaves and freemen rather than tribesmen; they had been in close contact with Chinese governments for centuries by then, and caused these states little trouble during this period.
Regarding tags, besides the independent states the only ones I would recommend adding are ones like Zheng, Teng and Cao, small but densely-populated former states of the Central Plains who had been important in the fifth and fourth centuries BC, and Dai, a former kingdom of the Di in Shanxi which would briefly resurface as an independent state under Xiang Yu's feudal system.
South China
Although South China only really has one independent tag in 304 BC, its cultural and political setup is unique and arguably more complex than the one in North China. The Sichuan basin, only brought under Qin rule in 316 BC, was formerly home to two non-Chinese states, Ba and Shu, each deserving of their own culture and tag. The northern part of Chu, centered around modern Hubei, was home to a distinctive Chinese subculture (both Liu Bang and Xiang Yu would hail from this Chu culture), but most of their sprawling territory to the south and east was much less Sinicized.
The eastern territories in modern Jiangsu and Zhejiang had been home to powerful and sophisticated non-Chinese states, Wu and Yue, and had only been brought under Chu rule in the 330s BC; besides some Chu citizens the population should be made up almost entirely of freemen and slaves of Wuyue culture (there could hypothetically be some Wuyue citizens, but even in the old Wu and Yue kingdoms the elite was quite sinicized, and should maybe be rolled into the Chu Chinese culture for simplicity's sake). The southern territories, around Hunan and Jiangxi, had civilized settlements of Chu people (or, in North Jiangxi, Wuyue people) at favorable points along the major rivers, while the rest of the land was inhabited by various Baiyue or Nanman tribes, a number of whom ought to have tags available to them.
The most important takeaway here is that, in Sichuan and the Lower Yangzi, there ought to be substantial populations which are both "civilized" and culturally outside of the Chinese group, each with a tag available to them (Ba, Shu, Wu, and Yue).
The Far South
The parts of modern southern China that were outside the reach of the Warring States- modern Fujian, Guangdong, Guangxi, Guizhou and Yunnan- plus the Red River valley of modern Vietnam, which would come under Han rule in later centuries, are not well-attested in this period but we know enough to broadly draw some cultures and tags. Although all of the tribes of the region were referred to by Chinese under the umbrella terms "Baiyue" and "Nanman", we now know that they included peoples from at least four different language families- Tibeto-Burman, Tai-Kadai, Austronesian (related to Taiwanese aborigines, Malays, Polynesians, etc) and Austroasiatic (related to Vietnamese and Cambodians).
Naturally, the fact that we know the area was quite diverse culturally, and yet we only have a limited set of imprecise names to describe them with, means this area calls for some creativity. These cultures, once drawn out roughly, can be grouped together on the basis of a good guess of their linguistic identity, or they could be grouped together on a geographic basis.
Although these areas should be overwhelmingly tribal, low-centralization, and low-civilization at the start, by halfway through the game the region was home to a handful of strong regional chiefdoms and even something like a proper kingdom in Nanyue (Guangdong, Guangxi and north Vietnam).
The Steppe
Major nomadic peoples to the north of China included Iranic groups like the Yuezhi and Wusun, proto-Mongols like the Donghu, Tibeto-Burmans like the Qiang and, of course, the Xiongnu, whose linguistic identity is not well-understood. Meanwhile, Manchuria and the Korean peninsula were inhabited by proto-Korean tribes who would found Gojoseon, Buyeo, and finally the Korean Three Kingdoms. For this region, I recommend grouping the Iranic tribes together with the Scythians and other related cultures already in-game; the Qiang nomads could be grouped with the Rong and Di, some of whom have given up their nomadic ways; the "other nomads" of various proto-Turkic, proto-Mongol, Yeniseian, etc origins (mainly Xiongnu and Donghu) ought to be a group; and then another for the semi-agrarian peoples of Manchuria and Korea.
Formable Tags and Union Cultures
The Han dynasty is one of the most influential and important empires in history, even giving the Chinese people their modern ethnonym, but the fact that the Han state came to be at all was a bit of a fluke and really can't be modeled with existant game mechanics. It also saw a blending of the Warring States regional cultures, a reinforced common identity, and a hardening of attitudes about the divide between Chinese and barbarians- but under current mechanics, we would see assimilation into the culture of the conquering State rather than a fusion into a new culture. Basically I think the best solution would be to set up a Han Empire formable tag, and a Han culture melting pot for the Chinese culture group.
Other strong candidates for formable tags are the Korean kingdoms, Kushanas/Guishuang for Yuezhi who conquer parts of Central Asia, and maybe some Easter eggs for difficult achievements (like reforming the Shang dynasty if you unify China as Song, ruled by Shang descendants).