This is very much an exercise in alternate history and assumes the japanese won at Khalkin Gol (the loss was the whole reason they abandoned it and moved to the southern expansion doctrine). So when the Germans would invade the USSR, japanese would move in too. The only problem is the right side of russia is pretty much empty and useless. And the USSR would lose because it could not fight a 2 front war.
So what would the japanese gain in the end? And was the northern expansion doctrine poorly thought out from the beginning?