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Gustav Samuel Leopold, Count Palatine of Kleeburg and later Duke of Zweibrucken (AKA this guy: https://en.wikipedia.org/wiki/Gustav,_Duke_of_Zweibrücken) was a paternal cousin of Charles XI and XII of Sweden, and the last male in the Swedish line of descent from Katherine Vasa. IOTL, Charles XII's sister Ulrika Eleanora seized the throne upon his death, then she handed it over to her husband, Frederick of Hesse-Kassel.
With any POD between Ulrika Eleanora's marriage to Frederick of Hesse-Kassel and Charles XII's death in 1718, what if Gustav had somehow managed to become King Gustav III of Sweden? IOTL, by 1718, he was married to Countess Dorothea of Palatine-Veldenz, but they had no children, and when she died in 1723, he married morganatically to Louise Dorothea von Hoffmann, with whom he also had no children.