So, ai realized that it's very fun to play a communist Japan in Victoria: Revolutions. Ai typically always go communist in the 1870s or so during the grand campaign. Very ASB.
Ai was thinking, though, that a communist revolution might've been possible in Japan in the late 1920s, early 1930s. Rather than a slight swing to the right, Japan could've seen a heavy shift to the left.
If, say, Japan had pursued a better chunk of territory for itself in the wake of World War I (refusing to withdraw from Russia's Far East, or from Shandong, or whatever), the Empire may have found itself in a bigger war than it experienced OTL. At the time, Japan was also suffering from internal upheaval similar to other industrialized countries, what with anarchist and socialist groups rising up and agitating and pushing for revolution. Korea was especially bad for this, and many Korean rebels were anarchists and socialists.
Japan was constantly being put under intense international pressure to back off whenever they had made gotten themselves a nice new chunk of territory, and that was a constant source of frustration for the Empire. Therefore, let's imagine they don't back off, even when they fail to get all their claims recognized in the Treaty of Versailles, and they then find themselves in a war with one of the Western powers, likely the United States. In this war, they find that, whule they aren't getting completely slaughtered, they are very much on the defensive. Taiwan is lost, as are all the Pacific islands Japan had grabbed from Germany in the war. At the same time, a general insurrection breaks out in Korea. In this situation, Japan is forced to sue for peace, probably brokered by its ally in Britain. They manage to keep the territorial integrity of the Home Islands (which would be impossible to invade), as well as their greatest colony Korea. But they back off from China and/or Russia, Taiwan probably becomes an independent republic (under heavy influence of Britain and maybe the States), and the States achieve the maximum extent of their Pacific island empire many years earlier than OTL.
For WHATEVER reason, Japan finds itself a lot less capable of liquidating the insurgent groups in TTL than OTL. (By 1924, the anarchists were crushed and the communists in hiding.) The situation is particularly bad in Korea, with the whole situation becoming Vietnam/Afghanistan/quagmire-esque, but there are Japanese groups in solidarity with the Koreans, and they are operating on the Home Islands.
At some point, it's the revolution! Ai see this being a long, bloody, horrible war, not ending until the late 1920s or early '30s, when the Depression would hae begun. Korea and Japan become a single polity, however, in some kind of pseudofederal arrangement. The anarchists are done away with, as in the Russian Revolution, but this revolution will still have a markedly less authoritarian element, in some respects. The economy is made syndicalist, for exmaple, rather than centrally planned.
The Japanese monarchy are not executed, but forced to live in internal exile, similar to the Chinese monarchy after the Chinese revolution. There are truth-and-reconciliation commissions set up for Japanese colonial rule over Korea, there are really big Korean and Japanese equality campaigns, Korean is taught in Japanese schools, etc. Koreans are integrated into the military, which maintains its very large role in the society. The new capital is probably Hiroshima, Fukuoka, or some other Japanese city close to Korea. The country has a very ambiguous relationship with Moscow.
Ai see Japan-Korea getting its economy largely functioning again, but largely via the same means as a fascist country: preparing for war. Japan will probably pursue its policies of imperialism in China just about as vigorously as it did as a conservative, borderline fascist country. (But this time, of course, to spread equality! And probably with support of Chinese revolutionaries, should they still exist in significant numbers by, say, the 1940s, when Japan-Korea is up and running again.)
Ai dunno how this would effect the Second World War. The whole thing might even be butterflied away, replaced with some other kind of world conflagration altogether. But, umm, ai see Japan-Korea making some kind of land grab in China, and probably not a whole further. And ai see the whole thing falling apart in the 1970s, 1980s at the latest.
Sketchy ideas, need fleshing out. What do you think?