I was looking at the cultures of east asia, a part of the map I usually pay very little attention to, and realized that Korean, Japanese and Chinese culture has been forced together in a single culture group. I found this very strange since, for example, India has been divided into no less than 4 different groups (west and east Aryan, Dravidian and Hindusthani). Now I'm fairly familiar with Indian history and I know that India as a cultural and political concept wasn't created until the British Raj in the 19th century, but I would argue that India having different culture groups is less justifiable than say splitting the Latin group, which is a mish-mash of supposed "Italian" cultures, while in fact Italy was just as vague a concept as India. North and South Italy was historically vastly different, and the remains of this difference can be seen even today in Italian politics, even more than 120 years after the unification. Keeping this in mind, and then moving ones gaze to east Asia... well I think you get my point.
Someone might argue that because east Asia shares a political history they could just as well be thrown in with the same culture, and if that's the role culture should play in EU3 then I'll buy that, but in that case you might as well make all of Europe into "European" culture. After all we all sprung from the ashes of Greek and Roman antiquity, and that connection is about as strong as the ties Korea, China and Japan has.
One might also argue that, if I get upset enough to start a thread on it being unjustified to put these distinct cultures into the same group, then why am I not attempting to kill someone over the fact that there is such a thing as the "African" culture group. Well the thing is that Africa is almost always destined to be colonized by Europe anyway. It serves no purpose what so ever to split Africa, because with the lack of centralized states at this time, it would result in a huge amount of groups. Many of these groups would only include a few provinces, all of which would be instant-converted to European culture at the time of the colonialism.
To get back to east Asia, I think that the three "Chinese" cultures (Manchu, Cantonese and Chihan) should be in one group, while removing Korean and Japanese and either putting them in separate groups, or possibly unifying them in one culture group.
I agree that putting them in all in the same culture group is not very historically realistic, but I'm willing to overlook it simply as a game balance issue (like Romanian being grouped with South Slavic, Hungarian with West Slavic, and all African cultures lumped into one group).
In terms of game balance, I could understand that the East Asians could have an easier time ruling each other than say, Southeast Asians or South Asians (the "chopstick nations" share Mahayana Buddhism, Confucianism, Taoism, and characters in common, while Southeast Asia minus Vietnam is all Theravada/Muslim, and South Asia is all Hindu/Muslim).
If I were striving for more realism, however, I would make for an East Asia with many more provinces, and divide them up the following way:
Make China its own culture group, either the Chinese or Han group, with various provincial subcultures. In general, each province in China is identified with an ancient kingdom or duchy, and the name of that kingdom is used to identify various aspects of the regional culture. For example, a kingdom named Yue formerly existed in the Cantonese areas, and Cantonese stuff is identified as Yue cuisine, opera, language, literature, etc.
As an example, I would make divisions like this: Min (southeast), Wu (east coast), Yue (Cantonese), Hakka (scattered), Xiang (along the Yangzi river), Shu (Sichuan), Qin (central China), Yan (Beijing and nearby environs), etc. That way, revolting Cantonese provinces would go to another Chinese state instead of Korea like current EU3.
I wouldn't put Korea and Japan in the same culture group, as historically their rules almost never overlapped, and it shouldn't be justified in-game that they have an easier time ruling over each other. I'd make Korean it's own culture group, like Basque. Revolting Korean provinces should not go to Japan, and vice versa.
With Japanese, I'd make a Japanese culture group that also includes Ryukuan or Okinawan (it's different enough from mainstream Japanese to justify its own culture), and depending on how many provinces Japanese is split up to, break up the Japanese islands into different cultures as well. In EU3, Japan is one unified country, invading Ryukyu from day 1 of the game, but historically, it was split up into dozens of factions in a centuries-long civil war. Only the power strong enough to unify Japan should be rewarded with a union tag that allows to rule over all the Japanese.
Manchus, I'd either put them in the Chinese culture group (since they did "form China" historically), or split off Mongol from Altaic and put Mongol, Kalmyk, Buryat, Dzungar, and Manchu into an east Altaic/Mongolic culture group. Historically, the Manchus had close relations with the Mongols, and intermarried with them, and had the same religion (Lamaist Buddhism), and even the same writing system, though the Manchus also became very Chinese during their rule of China. (I think in real life, the Manchus made an "Accept Cultural Shift" decision, without suffering the associated stability hit

). The Manchu issue is a bit tricky, Revolting Manchus could (and did) defect to either the Mongols or China, while the Mongols wouldn't defect to China. Chinese would defect to Manchu (and did, see Qing Dynasty), but not to Mongols, so I think the Chinese culture group is more justified.
The last I would split off is Tibetan, since it's not justified at all that they share a culture group with the Burmese other than that they're both Sino-Tibetan languages. They historically never ruled each other, nor would they have an easy time ruling each other (different lifestyles, religions, ruling philosophies, writing systems). I'd split off Tibetan into a "Himalayan" culture group, that includes central Tibetan, Bhutanese, Ladakhi, Balti, Sikkimese, Sherpa, and various other Himalayan cultures. Justification? They all have a Himalayan lifestyle, same religion (Tibetan Buddhism), writing system (Tibetan alphabet), and their languages are closely related enough that perhaps if they were one country, they'd all be considered dialects of Tibetan. It's very plausible that Ladakh revolting from Delhi can and should go to Tibet if it can't get independent.