I agree, it is criminally underrated.
Rather than Japan, I would much prefer focusing on the Sui/Tang, with a DLC that focuses on their invasions of Korea and the wars that involved China, the kingdoms of Korea, and the Japanese Yamato. Then another DLC later that uses that map for a later period of Japanese history, the earlier period of samurai that so rarely get covered, the Minamoto and Fujiwara. It could also fit in well with the period, as the Mongol Yuan invaded the Japanese after a century of China and Japan getting involved in court politics.