An idea I had.
The mongols invade Japan and brutally conquers the island, perhaps destroying the imperial family and dividing Japan (more than it was in otl) into warring clans. Japan's isolation from the rest of the empire however means that the mongols find it easier to just collect tribute from the clans, ruling through a middle man (shogun?). This middle man eventually manages to become very rich and powerful, and throws out the Mongols as well as unifying the rest of Japan. Japan now has a more unified government and fears another invasion from the sea (like Russia did from the steppes), and becomes obsessed with conquering the Manchurian/Siberian/Chinese coast to ensure no new invasion can happen.
Feasible? I know it's a very rough sketch, but I thought it was interesting as it may kickstart Japanese imperialism a few hundred years prior to otl.