I was thinking a rump DPRK if the leadership was even more bugfuck crazy. That or a rump Empire of Japan post-WWII that preaches a return to isolation a la the Tokugawa era - the parts are there but I don’t know what spark would bring it about (perhaps a Downfall scenario with a shit-ton of firebombing and the US occupying Tokyo and everything south while the Soviets take Hokkaido. The rump Empire would be the northern third or so of Honshu, the DPRJ (or whatever Soviet Japan calls itself) would be Hokkaido, and the Republic or Japan would be Kyushu, Shikoku, and the southern two-thirds of Honshu including Tokyo.