Hmm, I have an idea for a POD for Gustavus Adolphus king of PLC if anybody would be interested:
December 1620 - mad nobleman Piekarski succeeds in his assassination attempt against Sigismund III, taking both the life of king and his son Vladislaus.
1621 election is held in the most nervous atmosphere up to date. Sultan Osman II has declared a holy war against the PLC, last year saw the defeat of Polish army at Cecora, death of Grand Crown Hetman and Great Chancellor Stefan Zolkiewski and captive of Field Crown Hetman Stanislaw Koniecpolski, Tatar raiders reaching Lvov and finally the death of king Sigismund and his son. Winter sejm decides that the election will be held not as customary in Warsaw, but in Lvov, so that the massed nobility may react to potential invasion.
Unusually, the election brings few candidates. Nobody really is that inclined to became a king of a country which will soon face a humungus Ottoman invasion. Usual French candidate is not present for Osman II refused to cease war if one is elected; chances of any Habsburs candidate are blown by the brutal repressions against Czechs after the Battle of Biala Gora. In this, Axel Gustafsson Oxenstierna arrives proposing candidature of Swedish king Gustavus Adolphus. While usually a Luteran candidature would be problematic, election held in Lvov increases participation of Orthodox nobility who would be happy to see a non-catholic on the throne. Gustavus moreover promises to bring 40 000 soldiers for the Turkish war - a proposal which is received very well by assembled nobles. What swings the scale is the withdrawal of internal candidature of Jan Karol Chodkiewicz, Grand Lithuanian Hetman and for the time being commander -in-chief of commonwealth army, who switches to support Gustavus. Thus on may 3rd 1621 the first luteran rex Poloniae, magnus dux Lithuaniae, Russiae, Prussiae, Masoviae, Samogitiae, Livoniaeque, Smolenscie, Severiae, Czernichoviaeque is elected
Thought about the POD? I think it would require GA to win some convincing victory in 1610s against Russia to reinforce his credibility as a commander to be more plausible.