Polish is naturally easier. The Dukes of Brandenburg-Prussia paid homage to the Polish Kings. I'd say the best option POD would be during the George William.
Have his son, Frederick William, die from illness, an accident, in the Thirty Years' War without children, etc. Thus his eldest daughter, Louise Charlotte, would inherit, and she was married to the Duke of Courland. I'm not sure of the succession of the Hohenzollern at this time, but I'm pretty sure George William only had a single brother who died childless. No other Hohenzollern would have a direct claim, as George William's father had married the daughter of the previous Duke of Prussia that was from an unrelated branch of the Hohenzollern. Not sure if Louise Charlotte and her husband would be accepted in German Brandenburg, so maybe that half of the inheritance would go to George William's other daughter, who was married to a Landgrave of Hesse-Kassel. With a Prussian-Courland union entirely under the Polish crown, you could see Prussia becoming Polish over the centuries.
The only non-Polish option I can think of would be Sweden, and that is harder. Have Gustav Adolphus of Sweden and his wife, George William's sister, have more luck with children. Have the original Christina not die as a babe, and their stillborn son born healthy. The OTL Christina then doesn't become queen. George William remains Lutheran, and thus he forms a better relationship with his brother-in-law than King Catholic Sigismund III of Poland (who still claimed the Swedish crown that he'd lost to Gustav's father). So Gustav receives Prussian aid during the Polish-Swedish War of between 1626-1629. Sweden emerges victorious in the war, taking Danzig, Frederick William dies in the fighting, and Sweden signs a peace treaty largely OTL except Poland acknowledges Ducal Prussia passing to Gustav's son instead of George's eldest daughter. An Alt-Deluge thus becomes a Polish attempt to regain Prussia, and a devastating Swedish victory cements Ducal Prussia ends up part of Sweden's Baltic Empire and the polish population is ejected and replaced with Swedish settlers. Several centuries under Swedish rule follows.
Definitely a stretch though. The first one is simpler.