During the 1700s Poland became effectively a vassal state of Russia, and many thought it would eventually be absorbed into the Russian Empire. What disrupted that in our timeline was the rise of Prussia, who managed to stymy this and instead have a partition.
But what if Prussia gets dismembered in the 7YW. What would then happen to the PLC? Would Russia outright annex it? How and when would they do this? Presumably other countries would object, but I can't see France and Britain sending troops to stop it happening, and Sweden & Austria wouldn't have the power to stop it on their own.