Apparently, before he died in 1566, his uncle, King Sigismund II Augustus of Poland considered him as a successor. What if Sigismund of Brandenburg didn't die and was elected King of Poland? This was before the Hohenzollerns converted to Calvinism. And before the Vasas, the Counter-Reformation and the Deluge, the majority of Polish Protestants were Calvinists. Lutheranism was seen as too foreign and German.
Then again, would Sigismund have even been accepted in the first place?