One question, maybe a bit vague of difficult, but does China have any "sub-nations"? I'm not talking about small minorities, but rather parts of the country that feels like another nation than the other part, yet still accepting a full part in the country?
I can only think of the difference between Mandarin or Cantonese-speakers, would they feel this difference?
In many countries, there are these "nations", and the feeling does not have to be very strong - for example Spain has Galicians, Catalans, Andalucians - they all agree they're Spanish, but they also feel strongly as these "sub-nationalities"
The danger with sub-nationalities is of course separatism, and Catalans in Spain have, as of late, been requesting independence
Another example I can think of in China are the "five nations", Mongols, Uyghur, Manchu, Han - and Tibetans? But! How relevant are these nations anymore, since they all would rather fall under minority? Does anybody think of China as "five nations" anymore?
I guess I could rephrase as: How many major identities exist in China, apart from "Chinese"?
(I am working on a comic book, and it is set 2000 years in the future. I am mostly planning the background world so far, and I broke up the big superpowers into many smaller countries. USA, Russia, China - and this was when I first realized I had no "hints" for China, other than the obvious autonomous regions, but not for "China proper". The various identities of American states - at least regions - is pretty evident, but for China I didn't find any - or I just don't know China well enough. I also could hardly find ANY sub-national flags, while Russia and USA have plenty of flags for all of their sub-national entities, states, republics, etc.
If you could, I'd appreciate if you took a glance at the map I've made, to see if my "future broken China" makes logical sense. And take no offense for China being broken, I broke every country, I threw Norway into a civil war

)
India is another one with no sub-national flags to find. It could be on purpose, as a means to avoid separatist symbolism and such)
Sorry if I ramble a bit here, lemme know if I'm not making sense
Wow, this one is difficult. I can hardly tell you "how many", as there are "too many", but let me tell you what I know.
Firstly, about the nations, it's true there are 5 biggest "Autonomous Regions" in China, and they are,
Zhuang Autonomous Region (in Guangxi Province)
Xizang/Tibet Autonomous Region
Hui Autonomous Region (in Ningxia Province)
Mongolia Autonomous Region
Xinjiang Uygur Autonomous Region
If you add Han nation, there will be 6.
However, it's not that simple. If you have seen my post about my journey to Yunnan Province, you will noticed that I mentioned a Nation "Dai", they don't have an Autonomous Region of their own, but they have an "autonomous state", which is smaller than autonomous region. The nation Dai has their own language and writings, which is very similar to what is used in Thailand. In fact, the nation Dai can be regarded as the ancestors of people now live in Thailand.
And the nation "Yi", "Jing Po", "Miao" also have their own autonomous state, although they are all inside Yunnan Province. There are 8 autonomous states in Yunnan, and it's only one province of China. And there are even more "autonomous counties" in China, which is even smaller than autonomous state. And there are some small races which have not been officially identified as a "nation", like "Ke Mu" people I mentioned in the journey post. Then you may see why I say it's very difficult to tell you "how many" are they.
Then we can talk about the problem you find about people who speak Cantonese. In the province of Guang Dong (Canton), there are 3 major languages/dialects, Cantonese, Hakka and Teochew, I will skip the less important dialects, as they are also "too many". People who speak different dialects in Guang Dong live in different area of Guang Dong in tradition, and they do feel the difference between each other. If in a university in Guang Zhou, a student who speaks Hakka has some serious dispute with a student who speaks Teochew and they decide to resolve it by

, it's very "normal" they both talk to their “townsmen association” in the university, gathering their townsmen, who speak the same dialect, to fight together. It will be some scene and really happened a lot in about 20 years ago

I've graduated for a long time so I don't know how often this thing still happen nowadays.
However, the language Cantonese is more like an "old official language" in Guang Dong, although it is also a dialect and the native language of some people who live in Guang Dong. In the past, like in about 200 years ago, when Mandarin is still not the national-wide official language, knowing Cantonese can be a symbol of "being well educated" in Guangdong. Just like nowadays if a Chinese can't say Mandarin, we can conclude that he is not very well educated, or at least did not work hard in school when he was a student.
Maybe I'm a little off the topic and let's get it back. As I explained above, a people who speak "Teochow" may have triple "identities", he will regard himself as "Teochow people" when he faces other native of Guang Dong, and he will regard himself as "Guang Dong people" when he faces people who comes from another province, and then he will regard himself as Chinese if he faces a foreigner.
But if we talk about sub-nation flags, I can say there's none of them (legally). I know Tibet may have it's own flag, which is forbidden in China and only be used by Da Lai and his supporters. But if you would like to use it in your comic map, I'm pretty sure your book will never been published in China
And as far as I know, only Tibet and the Uygur may have potential danger of claiming independent, the rest nations or dialect speakers don't have that kind of intention. "Offence of splitting the state" is also a serious crime in China. Any public discussion like "Tibet should be independent" is not allowed.