Interesting, but would never happen IOTL..
A win at Stalingrad is a long shot that doesn’t really guarantee winning the war.
There’s almost no reason to believe Hitler would adopt the trappings of a hereditary monarch and declare himself an Emperor in the aristocratic tradition. Sure he was the leader of the Reich, which has connotations of empire, but it’s a bit more complicated than that and he thought of himself more as the physical manifestation of the Germanic völk and it’s avatar on Earth rather than of being of a noble blood individually. Besides, he was never really committed to a family for himself, Eva Braun testified that he essentially viewed his entire existence as being a sort of servant to the “Germanic race”, not really something that lends itself well to declaring oneself and your bloodline to hold an imperial title en perpetua.
A somewhat minor point when compared with the rest of the timeline you outline, but Hitler had his city of Germania (to replace Berlin) obsessively planned with Albert Speer and his ‘palace’ was not going to consciously incorporate French and American public symbols. Much less ones dedicated to liberal republicanism.
Ignoring the fact that Hitler already had a wife and was not really interested in a Hitlerian ‘family line’ and would probably never have children had he survived, there’s a huge number of issues between a Germano-Japanese Union. Firstly.. how would that function? Even if the Japanese and Germans could or would actually divide formerly Soviet Siberia in half and actually establish a land border (which I strongly doubt), it’s entirely implausible. Where would the capital be? How would German and Japanese administration even cooperate? The administration of the Kokutai was very different from the a government run on the Führerprinzip and bureaucratic anarchy, and geographic space makes it even further impossible. The Japanese would have to submit their Emperor under Hitler or Hitler would have to submit himself under their Emperor which is not going to happen. Nazi racial distinctions towards the Japanese were more of a thing of realpolitik than any really held belief about the inherent qualities of the Japanese (Nazi racial pseudoscience was malleable at times to political circumstances, like with the foreign Waffen-SS volunteers). Also, even if the implausible establishment of world spheres for the Germans and Japanese came to pass, they’d more likely be rivals than indifferent, let alone friends or especially unification.
Lastly, it sort of implies a ‘fascist solidarity’ that existed between the imperial regime in Tokyo and the Nazis, or even the Italians for that matter. While IIRC some ideas of ideological kinship existed between Italy and Germany, I don’t believe this really applied to the Japanese at all. The Japanese did not conceive of themselves as ideologically fascist, I think it’s more of a historiographical classification, and the idea that Shōwa statism constituted something that can even be properly called fascist in the European sense is somewhat controversial. There certainly was an overlap of ideas though, for instance the concept of shinkoku dovetailed nicely with the Nazi application of metaphysical essentialism of a nations qualities. With that being said, regardless of historiographical constructs pertaining to the three ‘Axis powers’, I think to a Nazi or Japanese official the differences were quite pronounced. So the idea that the Japanese and Germans would even agree to produce a ‘fascist heir to rule the world’ has a lot of problems with it aside from purely material.
So yeah, essentially entirely ASB for a number of reasons. I’m sure it’s a fun read though, I’ll go check it out!