Make Japan an Empire and have the actual Emperor rule over it with the Yamato Dynasty; if possible, make a custom gov that ensures it will forever stay the Yamato Dynasty so long as it's a monarchy.

引用自 ♔M♔ Voltaire：

Make Japan an Empire and have the actual Emperor rule over it with the Yamato Dynasty; if possible, make a custom gov that ensures it will forever stay the Yamato Dynasty so long as it's a monarchy.

That sounds interesting, but the purpose of this mod is only to united the country for your use, not modify the country itself. Japan does not start out as an empire in the EU4 base game files, and does not have a unique government form (I think). As for the ruler, he/she is automatically generated when the game starts with a culturally realistic name.

Well I concur with the empire rank suggestion. The japanese emperor have existed since the earliest historial record in Japan - and he is not actually the secular ruler of Japanese territory before modern age, but more like the Pope of Catholic Holy See, who is the god's messenger and provides justifiability
of the entire feudal ruling system. And there is no such thing like "king of Japan" except being used as an insult in Chinese diplimatic files. You might know these two nations don't get along very well since long long time ago - anyway, the ruler of Byzantium in year 1444 was still called as Basileus of Roman Empire instead of Mayor of Constantinople, so it's also inappropriate to call him as king.