Interesting read. I'm not much of a historian, but i'm interested in what the trade good and population density compositions would look like if you could provide that later.
One of my books has very good population density maps for various points in Chinese history, but unfortunately I'm in the middle of a move and my books are all boxed up. I'll dig around online to see if I can find any good ones in the meantime.
Regarding trade goods, almost all of the goods mentioned in the
Trade dev diary are produced somewhere in or near China (exceptions are things like woad, glass, or papyrus). Silk was the major international export, since silk production techniques were virtually unknown outside of China until well after the end of I:R's timeline, but the volume of internal trade was very high and regional trade supported the growth of major cities in China just as it did in the Mediterranean (in the Warring States period, these were mainly state capitals; by the early Han they had the imperial capital Chang'an plus the Five Metropolises). Salt and iron production were staples of the economy and the state finances; stone was less important because construction was mainly in wood, rammed earth, or bricks rather than quarried stone and writing was done on silk, cloth or wooden slats rather than on papyrus. But it's particularly important that horses were only readily available in the band of grassland contested between the Chinese and the nomads. Han's westward expansion during the time of Han Wudi was motivated in large part by the need for horses to supply the Han cavalry in their wars against the Xiongnu.
It's also worth noting that elephants were indigenous to what is now southwestern China, and although I don't know of references to them being used in war during I:R's timeframe they certainly were a few centuries later. They could add a little more variety to warfare in the region.
Icedt729 might want to talk about the Xiongnu and Mongolia so I won't go into it too much, but the consolidation of Chinese states and their encroachment onto the steppe was the direct cause of the Xiongnu unifying and posing such a potent threat to the Han Dynasty; as a game mechanic I think it's practically ideal--the Chinese states will be fighting insularly for the first half of the game, and then unify only to find the Xiongnu have now emerged as a peer to whoever managed to win.
I probably should have led in with the Xiongnu, since including the eastern steppe would be just as important an addition was China proper and would arguably have more impact on the rest of I:R's map. I decided to leave them out just because we haven't had any dev diaries that deal with nomads yet, so I don't know if they're going to just be "regular tribals with horse archers" or if there are special mechanics planned for them. Either way, the Xiongnu are the prototype for all later steppe empires and were responsible for displacing the Yuezhi into Central Asia during the I:R timeline, leading to the destruction of the Greco-Bactrian and Indo-Greek kingdoms and the rise of the Kushanas in their place. The Han-Xiongnu rivalry was truly epic in its own right and regularly spilled over into the Tarim basin and Ferghana valley, which are already on-map.