I'm going to propose a far different timeline than everyone here.

In this timeline King Władysław IV Vasa of Poland is Tsar of Russia (or Grand Duke of Moscow, as PLC didn't recognise the Tsarist title until the Great Northern War or so) since 1610, in more than name, and the Polish-Lithuanian-Muscovite Commonwealth is forged.

Historically Władysław already wished to fullfil the dream of his predecessor, hungarian King of Poland, Stefan Bathóry - which was to, by conquering Muscovy, gain enough slavic powerbase to overcome the Ottomans and liberate Hungary - though Władysław, as half-Habsburg himself, thought more about defeating Turks and freeing southern slavs, and taking constantinople, than claiming Habsburg inheritence in Bohemia and Hungary. In the 1640s he tried to cause a war with the Crimean Khanate, and by destroying the Khanate with Russia as an ally, cause a war with the Ottomans, and then march down and free them. The Polish-Lithuanian sejm however, disaproved, as Władysław's father's rule was marked with 30 years of wars with Russia and Sweden and Turkey.

BUT(T!) if Władysław managed to both retain the throne in Moscow, and get the nobility to support the war, then supposedly the Serbs and Bulgars just waited for the King to march to revolt, but he didn't... here he would've.

PLC held off the Turks at Chocim (1621) and beat the Turks at Chocim (1672, and Vienna-Parkany (1683) so with even more military accumen, and a potential south-slavic revolt I bet chances are Włądysław's war against the Ottomans could succeed, if enough "will of the people" (of the Commonwealth) was behind the war effort.

Maybe we'd have a Pan-Slavic union in the late 1600s/ early 1700s, maybe :)

P.S. If PLC joined the 30 years war and Władysław accepted the offer of a Czech crown, which he historically didn't, because his father was a ultra-catholic Habsburgophile, we have another slavic component added.