Welcome. It's nice to see another member interested in East Asian history.
To begin, although China, along with several outsider dynasties, namely the Yuan and Qing, along with possibly the Sui and Tang, which ruled from China Proper, maintained general hegemonies within East Asia during their existences, this was not necessarily the case if you look at the history as a whole.
From around 500 BC to 1270 AD, there was a general balance of power among North China, South China, (North) Central Asia (Xiongnu, Xianbei, Mongol, etc), (South) Central Asia (Tibet), Manchuria, Korea, and Southeast Asia (Nanzhao, Dali). The borders and entities certainly shifted over time, but at one point, the alliance against China was strong enough to bring the Sui down, while the Han fought the Xiongnu for 200+ years. Later, when the Tang attempted to extend its control further, it was destabilized due to the An Shi Rebellion, and eventually lost control of Central Asia, as generals in that region with access to large armies had become independent of the government.
The balance of power was weakened and shifted drastically after 1270, but continued occasionally soon afterward, specifically among China (Ming), (North) Central Asia (Northern Yuan, Zunghar, etc), (South) Central Asia (Tibet), Manchuria (Jurchen/Manchu), Korea (Joseon), and Southeast Asia (Vietnam, Champa, etc).
Anyway, the map as a whole looks relatively reasonable, but assuming that the PoD occurs between 1350 and 1600, a few changes are probably necessary. Given the population within each region, the Ming should not exist in that situation, due to the populations within each region. However, China can probably be divided into a northern and southern entity in ideal conditions. The Mongols might retain that empire for a while, but succession issues would potentially destabilize the state, while Russia would probably attempt to take most of the western regions. Tibet and Uyghur look reasonable, although the three Central Asian states will probably frequently clash with each other.
Joseon looks reasonable, but essentially requires butterflying the Manchu away, as the Jurchen/Manchu strongholds were in Southern Manchuria. As a result, many of the clans would be assimilated into Joseon, and although any remnants might attempt to escape north, they would probably not establish a state due to the geography and climate. Meanwhile Joseon would probably attempt to expand into the rest of Southern Manchuria within a century or two before 1800 or so. Joseon's capital is also located too far north, as the population in that region would not be as established, so it should probably be located in or near what is now Pyongyang.
Japan would need to first consolidate its holdings in Hokkaido and the Ryukyu Islands for at least a few decades around 1750-1850 before attempting to expand into what is now Sakhalin and Taiwan, while the Zhou will not be happy with losing the latter. Having it expand further into the Americas is close to impossible due to the large distance involved, although it could theoretically establish very minor settlements before they are eventually incorporated into other states.
