The Nazis had just been content to control and consolidate what they had by the end of 1940? By not invading the USSR they could have concentrated their forces on defending Europe against any British incursions by land and air and future invasion even if the US had entered the war. They could also have concentrated more attacks on Britain itself. Would an invasion by the Allies even have succeeded?
Its pretty clear that Stalin's aims were not to stay within his 1939 borders. Just how far he would have gone, I'm really not studied up to go that deep. What would be interesting to see is how the Soviets would have faired in going on the offensive without the lessons learned in defending against the German invasion. It would also be interesting to see how the Soviets would have faired against the Germans unweakened by a disastrous campaign.