Japan-related TLs usually focus on the Sengoku or Meiji eras, which is disappointing since there's a lot of PODs regarding the Emishi and the area today known as the Tohoku region in earlier times. The Emishi were basically a collection of Japonic-speaking peoples (including refugees from Izumo, the most powerful non-Yamato state of early Japan) alongside at least one Emishi tribe who were Ainu speakers. The Yamato used both military and diplomatic methods to conquer them, and the latter produced many of northern Honshu's leading clans like the Abe clan, Oshu Fujiwara, and the Andou clan. All of these clans were descended from Emishi rulers coopted by the Yamato, and they ruled semi-independently with just as many links to China, Korea, or northern Tungusic and Ainu peoples as they had with Japan. Hiraizumi, capital of the Oshu Fujiwara, was once among the largest cities of Japan.
The Andou clan, successors of the Oshu Fujiwara, are just as interesting but are even more obscure (English language sources are lacking). They held the title "Ezo kanrei" and thus nominal rulers of "Ezogashima" (essentially Hokkaido, Sakhalin, and the Kurils) and controlled the large city of Tosa (some sources call it Tosaminato) that like Hiraizumi, was an important gateway to China, Korea, Manchuria, and the northern islands. They were also of Emishi origins and their descendent clans like the Akita are unique in not claiming descent from Emperor Jimmu. Their armies at times included many Ainu (both Honshu Ainu and Hokkaido Ainu) and their navy was supposedly one of the strongest in the Kamakura era. They seem to never have quite made it since one of their leaders was too aggressively Buddhist and provoked decades of Ainu rebellions that coincided with several divisions within the clan (which the Kamakura shogunate/Hojo clan could not resolve with their own intervention which would lead to their fall several years later). While they were still powerful and wealthy in the early Muromachi era, their clan ended up fairly fragmented and newer clans rose to take many of their former positions. Tosa was destroyed by a tsunami and later silted up.
To me this seems like this area had the best chance to keep Japan divided and foster a unique culture that is one part Sino-Japanese, one part indigenous (the Oshu Fujiwara for instance were genetically Japanese, but at one point treated their dead like the Ainu). To me it seems plausible that such a state could extend to include Hokkaido and maybe even further--perhaps a Mongol-ruled Ezo with the Andou clan as hereditary rulers?
They were fairly successful in the late Republic and early Empire and apparently even displaced Berber speakers in some areas, but increasingly assimilated to either Roman or Berber culture by the late Empire until they vanished sometime after the 6th century. My idea was to give them Malta.