I've always wondered how long Poland could last if they did everything right and the Germans did everything wrong. I know that Poland apparently mobilised its army, then unmobilised it, then mobilised it again late in August 1939, while Germany postponed the invasion by a few days allowing multiple new divisions to be ready for the attack.
Also, what if the French decide to pursue the Saar offensive with some determination?
If Poland can hold on long enough, the Soviets could become nervous about attacking - what if they are held back by the Poles and the Allies decide to declare war on them too? The Soviet invasion depended on nobody caring about Poland's fate at that stage.
Goal: Poland survives, in the form of a recognisable military formation that hasn't surrendered in Polish territory, until January 1st, 1940