Toppling the Emperor in a democratic revolution is impossible- the Emperor was too much a symbol of nationalism and willing to play the puppet of whatever group grabbed power. Any democracy movement would take the simple solution of figurehead monarch- the monarchy was, after all, popular.
A social revolution is much easier- the same divisions of society exist in Imperial Japan as everywhere.
Have the Japanese lose the Russo-Japanese War and the Russians imposing terms severe enough to cause widespread suffering. Then the elite tries to push the burden of the defeat onto the lower classes. Could be enough to spark a social revolution
Won't work after WWII because the American occupation would lead any revolt to be nationalistic and the Emperor the perfect leader of the revolution