The challenge is to try and get the Japanese Monarchy formally abolished. It doesn't matter how democratic the Japanese Republic is; it can be an ultra militarist fascist state, a liberal democracy, a one-party dictatorship, an oligarchy of aristocrats, a federation of anarchist communes, a Soviet Republic, etc. anything goes so long as the Monarchy has been officially consigned to the ash heap of history (although the former monarch and royal family may continue to have some degree of de facto power). There is a catch, however. The abolition of the monarchy can't be as a direct result of a post WW2 US/Allied/Soviet occupation of Japan, although the groundwork for the abolition may be laid during an occupation.
It might seem an impossible task, but remember that 100+ years is a long time, and if you had told the average Russian peasant at the outbreak of WW1 that in less than half a decade Russia would be a republic, they probably wouldn't believe you.
It might seem an impossible task, but remember that 100+ years is a long time, and if you had told the average Russian peasant at the outbreak of WW1 that in less than half a decade Russia would be a republic, they probably wouldn't believe you.