I am not sure if the Mandate of Heaven has been incorporated well here, as according to
Wikipedia the Mandate of Heaven, was in China, so I am not sure why Japan, or such as that of Dai Viet, would be able to take it from China, as they simply had different beliefs than the Chinese, and I highly doubt the Chinese would like a emperor thst is not Chinese. Often times the Chinese themselves would overthrow their emperor, and install a new one, whom was more "just" than the last dynasty, whom had lost the Mandate of Heaven. Through yes, I know the feature gives something to Far East Asia, but still, I don't think it is depicted as very well, from what I have seen so far in screenshots. Perhaps make it so a certain part of your population has to be from the Chinese culture, with their religion as well, in order to claim the Mandate of Heaven, as then it would very well make much more sense, than the DLC's current state, as I see it.