So I know the Wettins ended up as kings of Poland for a time, but could the same happen to Brandenburg-Prussia? Conversion would probably be even less of an issue for the Prussians- as the Electors were Calvinist, as opposed to the majority-Lutheran Brandenburg territories, and after all Saxony itself remained Protestant even as the Wettins were Catholics.
A union with Brandenburg-Prussia accomplishes a few things, depending on when it happens. Prior to Frederick the Great Brandenburg-Prussia is a major power, but not a great power- they are second only to Austria within Germany, comparable to, say, Bavaria in the wake of Vienna- but by becoming kings of Poland they eliminate the issue of having a territory nominally as a fief of Poland and gain a Royal Crown. It also gives the Polish king a strong power base outside the realm (which would IMHO be the biggest objection of the Sejm- although they allowed the Saxon union). Post Deluge it also means almost certain war with Russia as they viewed the Commonwealth as their sphere of influence and would not look kindly upon the Prussians muscling in on them. That said a Russian war could work to their advantage, if the Prussian monarch is able to offer an army, if this nation could use say the Great Northern War to force such a union. IIRC one of Frederick the Great's brothers also contemplated making a bid for the throne though Freddie opposed it.
A union with Brandenburg-Prussia accomplishes a few things, depending on when it happens. Prior to Frederick the Great Brandenburg-Prussia is a major power, but not a great power- they are second only to Austria within Germany, comparable to, say, Bavaria in the wake of Vienna- but by becoming kings of Poland they eliminate the issue of having a territory nominally as a fief of Poland and gain a Royal Crown. It also gives the Polish king a strong power base outside the realm (which would IMHO be the biggest objection of the Sejm- although they allowed the Saxon union). Post Deluge it also means almost certain war with Russia as they viewed the Commonwealth as their sphere of influence and would not look kindly upon the Prussians muscling in on them. That said a Russian war could work to their advantage, if the Prussian monarch is able to offer an army, if this nation could use say the Great Northern War to force such a union. IIRC one of Frederick the Great's brothers also contemplated making a bid for the throne though Freddie opposed it.