Missed this thread a while ago, but here’s a suggestion born from previous discussions of alternate Jagiellon marriages...
Jadwiga of Poland and her daughter, Elizabeth Bonifacia, survive, but the birth is so hard that Jadwiga can never bear another child. The Polish-Lithuanian union passes into the hands of whoever marries Elizabeth. In this scenario, Eric of Pomerania, ruler of the Union of Kalmar. This creates a Polish-Lithuanian-Scandinavian Union where Poland is the demographic heavyweight. During the fifteenth century, this union absorbs the remaining German states in the Baltic, and its long Atlantic coast gives it a window to overseas conquest that Poland never had IOTL.
The possibility for adding Hungary to this union also exists, though it gets ungainly and entangles the union in wars against the Ottomans. In retrospect, disentangling from the Dzikie Pole until the 18th century or so would be better—avoid the demographic and financial drain of eternal warfare against the Tatars. Let Muscovy’s citizens go into slavery instead, while Poland-Lithuania-Scandinavia’s settle America.
So much for starting the empire. Developing the financial and legal institutions to finance world domination would require then creating a mercantile class. IOTL, those were in large part German burghers in Poland. Maybe that state of affairs continues—the Commonwealth might eventually look a bit like a caste system, with a Polonized aristocracy, and a German/Scandinavian burgher class. OTOH, wirhout Ruthenia, maybe the magnates in general would be weaker, and a King could emerge who could strengthen the burghers, bring them into the higher ranks of society, and bring the aristocracy to heel—possibly forming the union of capital and aristocracy that characterized Britain IOTL.