Simple: August III/Friedrich August II, the King-Elector, dies before his father. Let's say 1720. His only child is about to die and his wife, Maria Josepha von Habsburg, is cast back onto the European marriage market. I'm not particularly fussed about who she marries, because her uncle Charles still has time to produce a male heir, so the whole Pragmatic Sanction thing doesn't matter that much yet. But feel free to speculate.
So when August II dies in approx. 1733 with no legitimate issue, the Electorate of Saxony goes to his second cousin Christian, Duke of Saxe-Weissenfels, who was a bit of a spendthrift, but won't do too much damage, dying half a decade later. His brother succeeds and may produce surviving sons ITTL. If not, there are a couple more junior lines, which IOTL died out before 1760. If this happens, I assume the Duke of Saxe-Weimar-Eisenach becomes the new Elector. So what does the future hold for the House of Wettin?
The biggest effects would be felt in Poland. Without a convenient son to be placed by force on the Polish throne, do the Russians still present an opponent to Stanislaw Leszczynski? If so, who? And if not, does the War of the Polish Succession still happen (under another name, obviously)?