I think to make a WWII game that is realistic, balanced, and fun to play and has a historical starting point you need to give it an earlier start and more vibrant political rules and victory conditions. You could start the game something like 1938 and have a buildup & influence period of a couple of rounds. Each major power should have its own victory conditions. For example, if USSR wound up conquering all of mainland Europe, that would be a total victory for the Soviet player, a total defeat for Germany and France, and a defeat for UK and USA.
Wouldn’t you then end up with the USSR attacking Western Allied troops? Otherwise i like the idea, although I guess a 1 September 1939 start date would do fine.