Given the History of the Touhoku region OTL, it's probably going to take 1-2 centuries to completely consolidate Hokkaido. Sakhalin and the Kurils aren't going to be an option until more than halfway through that. Anything on the Mainland itself would be downright retarded.
Maybe the Mainland might not be so retarded FDW. I got to thinking. One thing about the Japanese at this time is that their trade goes much farther than their military reach. If the Japanese subdue Sakhalin (a logical step after Hokkaido), at the northern end, they are across a 10 mile strait from the mouth of the Amur River.
It would not be terribly difficult for Japanese coastal galleys to row up the Sea of Japan coast to the Amur mouth. Once there, being flat botomed, they could navigate the sand bars that are the bane of keeled vessels attempting to sail up the mouth of that river and start to row up the Amur, which has a relatively low population of Doyon, Lamut and Evenk people. How much the Japanese would want to actually conquer the people of the lower and middle Amur and how much simply trade with them would be unclear. But eventually, one reaches the head of navigation on the Amur---in the territory of the Northern Yuan post 1388 (before 1388, it is simply the Mongolian side of the Yuan Empire). From there, the overland trade routes at that time run clear to Tanis or Kaffa on the Black Sea (when there isn't an epidemic of Black Death going on), to Persia and India, and to Russia, Poland and the rest of Europe. Japanese traders and samurai, particularly ronin can travel those trade routes.And they will be participating in the politics and events in Mongolia and at the ends of those trade routes and keeping Japan informed of them, even if Japan does not directly get involved in most of them.
See Wikipedia Northern Yuan
he Yuan remnants retreated to
Mongolia after the fall of Yingchang to the
Ming Dynasty in 1370, where the name Great Yuan was formally carried on, known as the Northern Yuan. The Northern Yuan rulers also buttressed their claim on China,
[10][11] and held tenaciously to the title of
Emperor (or
Great Khan) of the Great Yuan (Dai Yuwan Khaan, or 大元可汗)
[12] to resist the Ming who had by this time become the real ruler of China. The Ming army pursued the Northern Yuan forces into Mongolia in 1372, but were defeated by the latter under
Ayushridar (r. 1370–78) and his general
Köke Temür (d. 1375). In 1375, Nahacu, a Mongol official of Biligtu Khan (Ayushridara) in
Liaoyang province invaded
Liaodong with aims of restoring the Mongols to power. Although he continued to hold southern
Manchuria, Nahacu finally surrendered to the
Ming Dynasty in 1387–88 after a successful diplomacy of the latter.
[13] The Yuan loyalists under Kublaid prince
Basalawarmi (the
Prince of Liang) in
Yunnan and
Guizhou were also
destroyed by the Ming in 1381-82.
[14]
The Ming tried again towards Northern Yuan in 1380, ultimately winning a decisive victory over Northern Yuan forces around the
Buir Lake region in 1388. About 70,000 Mongols were taken prisoner and the Mongol
capital Karakorum was sacked and destroyed.
[15] It effectively destroyed the power of the Khaan's Mongols for a long time, and allowed the Western Mongols to become supreme.
[16]
Rise of the Oirats (1388–1478)[edit source | editbeta]
See also:
Four Oirats
In 1388, the Northern Yuan throne was taken over by
Yesüder, a descendant of
Arik Böke (
Tolui's son), instead of the descendants of
Kublai Khan. After the death of his master
Togus Temur (r. 1378–88), Gunashiri, a descendant of
Chagatai Khan, founded his own small state called
Qara Del in
Hami.
[17] The following century saw a succession of Chinggisid rulers, many of whom were mere figureheads put on the throne by those warlords who happened to be the most powerful. From the end of the 14th century there appear designations such as "period of small kings" (
Бага хаадын үе) for this period in modern historiography.
[18] On one side stood the
Oirats (or Western Mongols) in the west against the
Eastern Mongols. While the Oirats drew their side to the descendants of Arik Boke and other princes,
Arugtai of the
Asud supported the old Yuan khans. Another force was the
House of Ogedei who briefly attempted to reunite the Mongols under their rule.
The Mongols split into three main groups: western Mongols, the Mongol groups under the
Uriankhai in northeast, and the Eastern Mongols between the two. The Uriankhai and some Borjigin princes surrendered to the Ming Dynasty in the 1390s...
There's a lot more history of conflict between the Mongols and the Ming in the 1400s. That history may not be relevant ITTL. If the Northern Yuan post 1370 establish good relations with Japan, they may have help against China and may not lose the war against China. They won't recapture China, of course. But they may retain their independence with Japanese help ITTL.