This is an excellent "What If"...without the invasion of Poland, how would the conflict start? Hitler had already bullied the Allies in annexing Czechoslavakia...would he have dared attack the USSR in 1939? I don't think he would do so...maybe he would continue Germanic expansion into Eastern Europe. That is, replace the attack on Poland with Hungary and maybe Romania. This would probably be the move he would make as it would give some Lebensraum (Hungary) and economic benefits (Romania) from the Ploesti oil fields. Of course, both countries became Axis powers after Germany attacked the USSR in 1941, but how would they react to Hitler's pressure in 1939. Would they succumb like Czechoslavakia or put up a fight. If they did fight, would this trigger WWII? France and England did not have any treaties with these countries. Could the USSR pull a deal like they did with Poland and split Hungary and Romania with Germany? Interesting scenario...
If Hitler did not make a move in the East in 1939, would he attack the low countries and France in 1940? I think he would leave well enough alone in the West and concentrate on Eastern European conquests.