I've actually been working on one of these! Here is a very preliminary TL that spans a few hundred years...basically, everything goes right for Poland (and Byzantium could also survive until today).
1444 Vladislav III of Poland, Lithuania, and Hungary (as of 1440) is victorious over the Turkish army at Varna during his Crusade against the Turks (instead of being slaughtered with his army). Over the next three years, he obliterates the Turkish Empire, which is as of this time still not all-powerful. He dies three years later than in OTL.
1447 Casimir, Grand Duke of Lithuania, becomes king after Vladislav as Casimir IV. During his reign he inherits Bohemia.
1573-1583 Maximilian Hapsburg attempts to take the throne of Poland, invading and occupying Bohemia. However, after 10 years of nearly constant warfare, Stephen Bathory defeats him in battle, and forces Maximilian to return Bohemia to Poland.
1594 After Sigismund III is elected King of Poland, and becomes King of Sweden, he not only reassures Swedes that despite the fact that he is Catholic, he will respect their Protestant beliefs, but he also makes sure his paternal uncle, Duke Charles, has a horrible hunting accident (in OTL this Duke Charles ended up rebelling and becoming King of Sweden himself).
1610 Sigismund III allows his son, Vladislav, to take the throne of the Tsars.
1632 At the death of his father, Vladislav IV becomes:
He builds a great Polish-Lithuanian-Swedish-Russian Commonwealth Navy, with two mighty bases on the Hel Peninsula, named Władysławowo and Kazamierzowo. His reforms make him an incredibly popular ruler, and he dies beloved by all of his subjects.
1648 John II Casimir, Vlasilav's half brother and cousin, is elected as king of Poland. Although he is generally disliked by the people, since he has a fondness for the "more civilized" refineries of Vienna and Versailles, he brings into Poland's sway the Kingdom of Portugal, which he inherited as a vice-king (the other vice-king apparently met with an accident). Portugal's vast empire at this time is well-protected, as later kings do not involve themselves in wars, and preserve the colonies.
1668 John II Casimir abdicates, due to his attempts to Catholicize Poland and Russia. Jan Sobieski is made King of Poland, Lithuania, Bohemia, Sweden, and Portugal, and Tsar of Russia. He spends many campaigns in the East, conquering the khanates and emirates of Central Asia.
1696 After the death of Jan Sobieski, his son, James Sobieski takes the throne without a struggle for power, because the Hapsburgs never became important enough to be the unquestioned and dominant power of Germany, and Prussia has never become a major power, since it is a fief of Poland, and East Prussia has long since been annexed to what is now being called the Confederated Empire.
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1910: The Confederated Empire ruled by James IV is one of the world's two superpowers. The modern-day nations ruled by James IV include:
Poland, Czech Republic, Slovakia, Hungary, Slovenia, Yugoslavia, Croatia, Bosnia-Herzegovina, Macedonia, Albania, Romania, Bulgaria, Lithuania, Latvia, Estonia, Finland, Sweden, Norway, Russia, Mongolia, Kazakhstan, Uzbekistan, Turkmenistan, Kyrgyzstan, Tajikistan, Armenia, Azerbaijan, Georgia, Iran, Portugal, Brazil, Uruguay, Guinea-Bissau, Angola, Sao Tome and Principe, Mozambique, Oman, Yemen, Sri Lanka, and Japan, as well as enclaves in India, Indonesia, and China.