This is a dope idea. What period of China would be the best for the game?
It would suck if the game gets banned in China, but you don't need to be from China to appreciate a game like that. I'm not from Europe, and I love playing European countries in paradox games.
So far as we known, only games covers anything of communism in China and also highly accepted by people got "banned". (Again, please note, players in China may feel offended by this word. As ban usually indicates something from a legal body, for example: German bans Nazi, which indirectly implies all Paradox players in China are acting something illegal while we are not. Reasons have been explained above. As some government running medias tried to frame Paradox players as illegal before, this matter can be... sensitive. Even on this forum, you can find posts about some Paradox players in China fear they may get arrested for just owning the game while I defended them.)
Thus, anything before 1800s are almost totally safe. The game may not get officially approved by the government.(As it requires Paradox to cooperate with Tencent again to get a chance to be considered to be approved just like what they did with Stellaris and City Skylines ) But, it will also unlikely to get attention from the government to make them try to remove you. So far as we know, the only Paradox game gets such attention is HOI4(Not even any previous HOI games), and there is no other Paradox games so well accepted by players in China to make government to interfere.(Still, what they did is just remove it from one online store which didn't even stop people from keeping buying it, playing it, streaming it, making tournaments of it publicly... Think about it, what will happen if someone post something about Nazi in German, that will be totally different.) Thus, consider it as an honor.

Usually, you will achieve a great sales before you get a "ban".
As for the interesting period of history:
1, Spring and Autumn/ Warring kingdoms which is the only true feudal period in China (everything after that is more highly centered bureaucrat)
During this period of the time. The Zhou Dynasty owns a small land in capital severing as the son/high priest of heaven while local petty kings rule their own kingdoms. (Or...you may call him Pope if you like

)
They follow a common moral code of Zhou, considering themselves as civilized against the barbarians around them.
Such code including things like: armies must let enemies to form their formations before fight each other. Soldiers shall not kill those already wounded enemies. Petty kings can only fight each other when having a CB... (A famous conversation during a war: The king of Sui: "I have no guilty, you can't war against me" The king of Chu: "I am a barbarian!" which basically tells only barbarian declare war without CB, So below to find out why Chu went to become "barbarian" and no longer being "civilized")
But, The kingdom of Qin whose lands are poor and right next to barbarians seeks to gain power, thus decided to abolish such moral code and went rogue. They made laws such as "collect enemy ears/heads to gain reward". Thus, other kingdoms got caught by surprised and fall one by one. Eventually, Qin dethroned Zhou Dynasty, thus, ended the Spring and Autumn and started warring kingdoms.
During the warring kingdoms, 6 other remain kingdoms also "reformed" old moral code trying to fight agains Qin and among themselves. It's a period of war/conspericy/diplomacy/reshape moral codes/find value of life/even...ideology.... Old faiths got challenged, new thoughts sparks among politicians. But, in the end of this legend, the "evil" won. Qin annexed the remaining 6 kingdoms. The king of Qin claimed being the first Emperor of China.(He also ordered to burn everything Confucius wrote, to show his deep disagreement to old values) But, shortly after his death, old nobles of 6 kingdoms overthrowed his heir and once again fought each other.....
So..basically, in CK2's turn. it's a period someone rage flipped the table and mudered Pope. And then, all Christian kingdoms turned into Pagen(or other faith) Tribes and no longer play by the rules.
Or in HOI's turn, everybody failed to stop Hitler, some of them even tried to turn Fascist, but still got annihilated, Soviet got facerolled, Britain surrendered, USA made a final stand but got slaughtered, Axis won, and then German also killed Italy and Japan. But, after that world conquer , German fall into a civial war with 7 or more ideology.
2, The Romance of Three kingdoms. Almost everybody knows now....
3, The end of Song Dynasty. It's a period when Song, Liao, Jin, Mongolia competed to be the emperor of China. Sometimes they even cooperate to fight their common enemies. Song evacuated to the south after losing war against Liao, but survived in Byzantine style, while north side of China keeps changing owners among Liao, Jin and finally Mongolia.
Koei made a Genghis Khan game about this period.
4, The end of Qing Dynasty. Revolutions, westernizations, civil wars. But, you are likely going to get banned. Because people can't stop but to streaming it everywhere and big brother will get mad.