Now I'll note what I see as possible China-related mechanics, in order of importance:
1. Concubinage (high priority)
In Chinese practice, rulers and aristocrats almost always had several concubines. The children of concubines could and did inherit, but were (theoretically) always second to any of the primary wife's sons. This system was partly responsible for China's byzantine court politics, with the multiple in-law clans and their various offspring and kinsmen all attempting to influence matters of policy and succession. For a western comparison, see how the polygamy practiced by the Argead kings in Macedonia worked out for them.
2. Fiefdoms (moderately high priority)
We already know of several types of vassals or dependencies in I:R, but what I'm proposing here are more like unremovable governors than tribute-paying subject states. I see two main applications for this mechanic:
- First, it models the fiefs handed out by Liu Bang to his supporters, which continued to exist throughout the Western Han even as their autonomy was gradually curtailed.
- Secondly, it could model the strong aristocracy in Chu, who were largely responsible for the state's failure to bring its full resources to bear against the other Warring States.
Essentially, the idea is that the transition from feudal order to centralized bureaucracy is a tricky, gradual one. The Qin empire gave out no fiefdoms and made no concessions to the nobility- as soon as the great conqueror Ying Zheng died his empire was shattered into the Eighteen Kingdoms. Xiang Yu overcorrected, giving too many fiefs to too many disloyal people- before long they destroyed each other too. Then Liu Bang finally got it right, giving out enough to satisfy his key supporters and the nobility writ large, but keeping enough for himself to keep military and political dominance.
From then on, the fiefdoms were gradually brought under ever more central control, first by removing all kings who were not of the Liu imperial house; then by splitting and shrinking them; and eventually by asserting the authority to appoint officials within the fiefdoms, essentially turning the title "king" into a sinecure with no actual power. In order to mirror this process, I argue it would be better for these fiefs to be a special type of territorial office or governorate within one tag, rather than as tags of their own in a kind of vassal relationship with the central court.
In balance terms, since Chinese unification is likely to happen more quickly than a Mediterranean or Central Asian one, the need to manage fiefs could enrich Chinese empire management and lower the risk of them simply steamrolling all of their smaller neighbors too early on.
3. Philosophical Schools (moderate priority)
As in the Hellenistic world, this was a great era for philosophy and social thought in China- the great thinkers Xunzi, Mengzi (Mencius) and Hanfeizi all lived in the early part of I:R's timeline- and these thinkers plied the Warring States in search of rulers willing to apply their philosophical principles to government. It would be a shame not to incorporate them in some way, whether as progressive sets of Laws or decisions, as itinerant characters seeking office in the States, as a kind of equivalent to CK2's education traits or in some other form.
One of Qi's great strengths was its Jixia Academy, where the state paid philosophers to take up residence, hold debate and put their works to writing; Wei was a great producer of Legalist scholars, including Shang Yang (who went to Qin and reformed their government); Qin produced no great thinkers of its own but did a fairly good job of poaching talent.
I would caution that conventional labels like Legalist, Confucian, and Daoist have had a long history and their use in, say, a Tang context or a Ming context may be very different from what it was in the Warring States, Qin or Han. This can make it challenging to determine exactly what they meant at any given time, and what their development signifies. I can elaborate on this further if there's interest.
4. Navigable Rivers and Canals (moderately ow priority)
Chinese rulers of the Warring States were keen to the advantages of water transport, so while only the Yangzi was large enough to bear large naval battles (as in the Chu-Wu wars before I:R or the Three Kingdoms period after), the Yellow, Wei and Huai rivers were all used extensively for trade and transportation. The Yangzi was also linked to the Huai by a major canal, built before I:R's start date expressly for the purpose of moving troops and supplies. Canal-building of a more limited scale went on throughout the period, supplying cities with fresh water, irrigating farmlands, and facilitating trade and communication. It would be nice to see these implemented in some way.
5. Noble Titles (low priority)
This would be primarily a flavor feature rather than one that impacts gameplay in a big way. Basically some characters would hold a noble title, bringing with it an income and privileges, while others would not, and their income and status would rely more heavily on their official careers. This reflects the distinction between the nobility, who held hereditary ranks that could only be given or taken away by the ruler, and the scholar-officials, who did not have any hereditary status and would fall back into obscurity if they did not continue to hold appointed office. In some ways this is similar to a distinction between true Patrician families and the families of "New Men."
There were five traditional ranks, from lowest to highest: nan 男 or baron, zi 子 or count, hou 侯 or marquise, gong 公 or duke, and wang 王 or king. By Warring States times the distinctions were minor, so there's no need to include all five. For Warring States kingdoms, the ruler himself is a King (wang), a holder of a fief (see my "fiefdoms" suggestion) could be ranked as a Duke (gong), and noblemen without fiefs would be Marquises (hou). For a united empire, the ruler is an Emperor (huangdi), the holder of a fief is a King (wang), and a nobleman without a fief is still a Marquis (hou). This could be made more complex but it's really not necessary.
A few more notes on Chinese feudalism: one, although Chinese households divided the family property on inheritance, a noble rank could only be passed on to the eldest son, with other sons receiving either a lower title or none at all. Enforcing primogeniture in titles but partible inheritance in property kept titles from proliferating uncontrollably and wore down the economic might of the nobles over time. Two, while fiefs were an important part of Chinese governance at the time, most noblemen did not hold fiefs in the sense of lands that were under their administrative/military control. Most were instead granted the tax revenues from a set of households- simply an income and not an actual position of authority. So in game terms a Marquisate is simply a salary (income going to them rather than to the state) and not a governorship or other political office.
In game terms, this means that the limited batch of families in the realm will include the royal clan itself, several persistent "noble" families who will linger around due to their ranks even if not gainfully employed, and several "scholar-official" families who rise up on merit and will disappear again if none of them are given an office within a certain timeframe. While the nobles themselves can be a liability, it will be necessary to have a certain number of titles and/or fiefs given out in order to maintain Legitimacy, character Loyalty or Citizen happiness (this need could possibly be reduced over time through technological advances, Laws, etc).
Additionally, I have a more "out-there" recommendation:
-Passive raiding/ border tension
This would be a global mechanic rather than a China-specific one. Essentially, under certain circumstances a province under the rule of an agrarian state that borders a province ruled by nomads will be subjected to low-level raiding. This would reduce happiness and income in the settled province while increasing it in the nomadic one. Good relations between the settled and nomadic state could reduce the effect (but not eliminate it), while bad relations would make it more extreme; the settled power could defend against it by building fortifications or attaching armies, while the nomadic power could increase raiding power by attaching troops or possibly setting a "raid" governor policy. The idea is to make these nomad-settled borders more complex and volatile, impacting China-Xiongnu relations but also Iran, India and parts of Europe.