A thought: suppose the Nazis accomplish this by not launching Barbarossa and concentrating their military resources on securing the rest of Western Europe instead, but Japan still attacks Pearl Harbor. Do the "Allies" even come to be? I'm sure Stalin didn't give a damn about defending the right to freedom and sovereignty in Britain, France, or any of the other conquered democracies, so perhaps the USSR leaves the Nazis alone as long as the Nazis leave them alone. If the U.S. military is the last one left standing with the resources and manpower to take on *any* of the Axis powers, is it possible that the U.S. still goes to war with Japan but sees Europe as essentially lost, especially if a more isolationist-minded President gets elected in 1940?
For a real wild card, would Hitler eventually, after consolidating his gains in Europe, insist that all British territory now belongs to Germany and send troops to Canada to set up a puppet government? It would be picking an unnecessary fight, sure, but he might still have been fanatical enough to try it, at which point maybe even the isolationist Americans would decide that they can't tolerate Nazis on their doorstep.