I dont know why Canada hasn't been mentioned as a country that actually did have a manifest destiny ideology.
But you could see a greater Iran Manifest Destiny between WWI and 1948. If Iran had played their cards right they could have incorporated many of the psuedo-Iranian populations such as Kurdistan, Armenia, Afghanistan, Tajikistan, Baluchistan, and even made a play to incorporate Shi'ite southern Iraq and Muslim India.
World War I might be a bit too late, but as late as the Qajar era, Iran could plausibly incorporate many of those regions.
Can you please elaborate on the traditional medicine part, please?
Can you please elaborate on your last sentence here?
OK; also, though, how many Japanese people would actually want to settle in Siberia in this TL?
Chinese traditional medicine (also practiced in countries influenced by China like Korea and Japan) has prized ingredients which Westerners like me consider strange. Nowadays, this infamously includes rhinoceros horn and other ingredients gained from endangered species. However, traditional Chinese medicine was always a constantly evolving discipline. If China had ready access to a continent with many new plants and animals, then practioners of traditional Chinese medicine would incorporate this knowledge. They did OTL with both Australia and the Americas as the horizons of the Chinese world were broadened, but I'm presuming this would occur both earlier, and with more Chinese, and Chinese as the dominant class, more intensely. If Australia was known to the Chinese earlier, then the relevant scholars would no doubt prize certain herbs or animal parts, maybe even moreso than OTL, and create a market for harvesting it in any country where people consider TCM useful.
By that I mean that Tokugawa Iemitsu was the one who instituted the Sakoku policy, Iemitsu being the 3rd shogun. Japan had a large trading network in Asia during the 16th century.
I see no reason why Japanese wouldn't want to settle in Siberia. Kamchatka has fine ports and trading opportunities with indigenous peoples, and the Lena basin is solid enough for agriculture, and further, can be exploited for the same reasons Russia exploited it--furs and whatever else to take from the locals. Same goes with all the territory between the coast and the Lena. It could be a place where Japanese, Chinese, and Russian influences combine (Russians were always trying to trade in Japan during the age of Sakoku). Agriculture, including subsistence agriculture, is very possible in most of Lena basin, and it's also rich in coal, gold, and other resources. Considering Japan's vast exportation of settlers OTL, I think Siberia offers plenty for the average Japanese peasant. The land involved would be, at minimum, Sakha, Magadan, Chukotka, and Kamchatka in modern Russia. I think much of Khabarovsk could easily be added.
Note that I'm not including Sakhalin because in this scenario Sakhalin would basically be another Home Island of Japan, no different than Hokkaido, as it very easily could have been as late as the 20th century.