Many campaign civs have their specially designed and more accurate flags, so I hope the same can be done for Asian campaigns.
Japan
Every Japanese faction in the Japanese campaign uses the Tokugawa clan emblem, which is only accurate for the player’s faction, while all the opponents were enemies of the Tokugawa.
For example, Ishida had his own emblem:
If giving each faction its own emblem is too much work, maybe consider the emblem of the Toyotomi clan, which most of Tokugawa’s enemies claim allegiance to:
China
The yellow dragon flag was only used as a national flag for the Qing Dynasty in its later years. One of the eight banners have a similar flag, but that still does not overlap with the time of the campaign (early Ming).
Chinese dynasties typically did not have a national flag or emblem. A common practice is to simply put the character of the dynasty name on the flag:
