Now obviously this idea requires some hand waves, since Japan has no reason to "Go North" in 1941, or any other year. However, I think it's an interesting scenario, with many repercussions.
So, Japan enters negotiations with Nazi officials in summer 1941 to invade the Soviet Union, and reach an agreement in late July. Due to the time it'll take to build up supplies and concentrate forces, the Japanese intend to commence offensive operations in late September.
The Kwantung Army in 1941 numbered some 700,000 men. The Soviet Far Eastern Front deployed close to a million men throughout the war, at minimum 600,000. It also contained a large portion of the Red Army's pre-war artillery, armor, aircraft, and transport. Due to distances few forces were transferred directly from the Far Eastern Front to the west, and it retained large supply depots. Essentially it was equipped to operate as an independent force. The Kwantung army by contrast had by 1941 been stripped if many of it's trained men, officers, equipment, supplies, and transportation. It was not prepared for intensive combat operations.
So, given from July to September to turn the Kwantung Army into a usable fighting force, what results could it achieve?