The question is how and when, you would need Poland to do much better, the Rus states to somehow never unifiy, all while keeping the romantic nationalist character of Panslavism still coming as OTL.
Only other alternative is Poland doing well in the Time of Troubles, but even with a proposed union there are some serious issues. Are you going to see bad blood between the Lithuania and Russia sections over what would be Belarus consider all the wars Muscovy and Lithuania had fought over it. Would such a union of large states even be possible, consider a theoretical would up the city of Astrakhan. Religion might be a powder keg since there's the very real possibility of an Orthodox Christian Majority, not to mention Muslims and Pagans in far eastern parts.
Wait, wait, wait. It's NOT how PLC worked at all... Majority of ruling class was Polish speaking. Initially PLC had three official languages- Polish, Lithuanian and Ruthenian. Last one was abolished in XVI century (or XVII?), because it wasn't used! Lithuanian language stayed official (but wasn't used at this point) until 3rd May Constitution abolished it as official language. Hell! Chmielnicky was Polish speaking (and his initial demands were about bringing murderers of his family to justice, not carving own state). It's not like one could go from Poland to Lithuania and find himself in entirely different world (although it would be correct for easternmost Ukraine- it was depopulated and lawless). Religion was also not an issue in PLC. Don't get me wrong- Catholicism was only "good" religion and other religions were chastised. But there was no religious unrest until XVIII century (Saxon kings) and even then it was about Lutherans, not Orthodox (there was also very big group of "Uniates-" Orthodox who considered Pope to be highest authority. And by "big", I mean significant portion people living in eastern provinces of PLC).
Making PLC into three part state isn't a good solution- it's not Europa Universalis. Nobles of PLC were abandoning their own languages, because they wanted to. No one forced them to do so.
My idea of the POD- Great Northern War, battle of Narva. Swedes destroy Russia (as they could), but get serious beating in the process (winter is severe in Russia this time of a year- ask Napoleon. Preferably Charles XII dies in the process

) and Augustus II wrests Livonia from them. It's ideal- Peter the Great loses control over Russia, probably sparking civil war (almost for sure), PLC isn't wrecked due to war (it was very important factor in coming years. 1/4 up to 1/3 of PLC population got killed or misplaced! It seriously pushed PLC back in terms of agricultural revolution) and Augustus II can strengthen his power. Now, only real threat (Russia is done for at least a generation, plus they have to modernize) is Prussia, and on its own it won't become a danger.