I like the basic scenario too...might need to play with that one if you don't mind.
It isn't totally original... If you do something with it, I'll be interested to see where you take it.
I got the idea from a Science Fiction book where there was a parallel "fake" Earth. There, German defeat in the Ardennes had simply lead to peace.
I had a need for a Northern development where Finland becomes pretty big. While I'm not totally pleased with the outcome, I'll stick with it for the time being. I'll post it now, to get it out of the way:
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By 1946, it was clear that "The Russians" actually had to be beaten on their own ground. It took some time....
... Most Finns would have been happy to get back the ground that they had lost, although there were those who also wanted the rest of Karelia, to create a Greater Finland (Suursuomi). Going for the Urals was stuff for morale raising songs such as "Uraliin" by Teijo Joutsela ja Humppa-Veikot:
As it turned out, all of the Soviet Union needed to be occupied to some degree, and southern Nations were not all that eager to take on the West Siberian Plain. By the end of 1952, USSR was occupied, and much of the North was held by troops consisting of recently recruited indigenous troops under Finnish command.
Once the fighting was over, the next problem was what to do with all the land and the various peoples living on it. With the progress of the war, nations further south had already been organised, or reinstated: Romania, Moldova, Ukraine, Slovakia, Poland, Belarus, Lithuania, Latvia, Estonia. The North had been under Russian rule for centuries but could not really be left that way. Russia needed to have well defined borders but the area had to be much smaller than before, to keep it manageable.
The first step in the north was a no-brainer for most: dump the Karelian areas and the Kola Peninsula into Finland's lap. The rest remained a United Nations protectorate until 1967, when talks among the peoples of the area and in the UN General Assembly resolved in the formation of the Finnish Federation, with its capital in Mikkeli, Finland. The new nation consists of the 6 States of Suomi, Karjala, Sámi, Perm, Yugra, and Mordva. In addition, the Nganasan Protectorate even further North East was assigned to the care of the Federation.