Funnily enough I discussed this not so long ago on a another board.
This issue will come up in 1527 (or was it 1528?) when Bona has her bear accident and miscarries Albrecht. This also comes after suicide-by-Turk as performed by Louis hence no male Jagiellons anywhere.
IMO Zygmunt is too Catholic to go Prot.
The candidates would be Zygmunt's nephews and grandsons. There are quite a few of them, some Catholic and some who could be persuaded that Cracow is worth Mass.
The most competent of the bunch is Albrecht Hohenzollern of Prussia
Yet another route is legitimising this fine fellow:
As we are in 1530 he could be married to a princess or duchess (in OTL more or less around this time his marriage to Anna of Mazovia was considered) and put forward for the throne. Forty years later there were desperate searches for even a bastard girl by Sigismund Augustus - not sure how serious, though.