the Ryukyu were mostly peaceful. It's capital being Shuri. Ryukyuan is the adjective, the official name of the state would have been Rūchū kuku 琉球國, but I think Ryūkyū or Ryūkyū Kingdom would be fine.
Ezo, the Emishi lived in Tohoku, which is in the northern part of Japan's main island, but they were pushed to Ezo (Hokkaido) Hakodate was the capital of the ethnically Japanese Republic of Ezo, but since the Ezo in the mod is the territory controlled by the Ainu/Emishi, Sapporo, could work it is the modern capital of the Hokkaido prefecture, and is from an Ainu word. The Russian names for the islands above Japan are from the Ainu language, so those could be states for it to expand into.
Korea could be represented by the three kingdom period of Korea, Goguryeo (Militaristic) and Silla (Militaristic?) callied with China and Baekje (Cultured) allied with Japan.
I am not sure if this is the information you were asking for though.