A semi-random thought hit me earlier today. Thinking about my original Zhongyang Dalu setting, I was considering ways to merge elements of various Asian cultures (and sometimes non-Asian cultural elements) into original cultures. That's basically what Gygax and Arneson did in their campaigns, just with mostly European historical cultures as the basis of their ideas.
In simpler terms, using discrete cultural elements as LEGO bricks and using them to construct a fantasy culture.
Then, something I'd thought about last year came back to me. In 1E OA (and in the 3E book's setting of Rokugan), social status is defined by the Japanese cultural caste system. Originally Confucian in origin (and possibly being influenced by the Indian caste system despite its differences), you get what I used in Chanbara. Nobles at the top, the "buke" (samurai caste) of warriors next, then commoners (in theory anyway), artisans, merchants (money for nothing [interest/mark-ups on goods] makes them rich but gets them no respect), and untouchables at the very bottom.
But in Korea, while there was a hereditary aristocracy, the Yangban, the real movers and shakers were people who could pass the Civil Service exam (in some eras of Chinese history this is also the case). Bureaucrats, functionaries, auditors, inspectors, governors, tax collectors, historians -- these were the influential members of society in Joseon Korea. Well, them along with the Military Service, which was also exam based.
It didn't matter how low-born you were (again, in theory), if you could pass the Civil or Military Service exam, you were made into an agent of the crown. In practice, low-born members like the Korean hero Yi Sun-shin, the admiral who helped defeat the Japanese invasions in the 16th century, faced discrimination and plotting by officials of aristocratic birth. But social mobility was possible.
I think this sort of social structure might be more conducive to an RPG setting than one where you're pretty much set in your social status at birth. Granted, in Japan's Warring States period, commoners who fought well could be granted samurai status. And in the same era and later in the Edo period after the wars were over, merchants with enough cash could purchase samurai status for themselves or more often for their children.
But in Korea, if you studied hard enough, you could rise easily through the ranks.
So, one thing to think about when designing fantasy OA cultures is to consider the social hierarchies and how people moved (or were prevented from moving) within them. The Mongolians had a more egalitarian society. Anyone who was a good warrior and leader could become khan, and you only remained khan as long as you were militarily successful or politically savvy. I'd need to study up a bit more on Philippine and SE Asian cultures, but I would bet the Thais, Burmese, and others may have had different structures as well.
Of course, in addition to politics and social structure, religion is important. And synchretism is the order of the day. There were many native animist practices in most regions of East Asia. There were Hinduism and Buddhism influences from India. There are Taoism and Confucianism (not originally religions, more just belief systems but made religious over time) from China. There is Bushido (again not really a religion but sometimes treated as one de facto) from Japan and Legalism from China. And in practice, they all blend together to some degree or other.
The Chinese conception of Buddhist Heaven isn't Nirvana, it's basically the Taoist conception of the realm of spirits and immortals, just with Buddha added in as the top boss. Shinto (animism) blends with Buddhism and Confucianism in Japan. In Korea, Buddhism was seen as an unwanted foreign influence by the government, who pushed a version of Neo-Confucianism as the primary philosophy for the people. Christian and Muslim missionaries were in China during the 8th or 9th century, maybe earlier.
Take elements from the above (and other belief systems, or made up elements that don't feel off) in various amounts, mix and match, and voila!
With the social/political system and religion of each culture spelled out, it's just a matter of adding some small, unique touches. And since this is fantasy, with magic and monsters and non-human people, adding in some unique touches helps make them feel different than humans. This is something I tried to do in the old 3E OA Zhongyang Dalu setting of mine, before I started retro-cloning Dragon Fist and needed a world with primarily Chinese influences.
As I said before, in most D&D settings, sometimes there are cultures that are very obviously drawn from one primary real world source -- the Known World setting has a lot of these, but other times the cultures don't handily map to a real world culture. And that 's a great thing for a game.