In regards of Hungary remember that in accordance with hungarian sucession law only the male descendants of Vladislaus were his heirs to the throne of Hungary. His broter was not. If he dies without a son the nobles are free to elect a new king - and they do not need to consider in any way or form the relations of the old king (this of course doesnt mean they wont).Vladislaus II had a stroke in 1504 which, given his age at the time, very well could have killed him. Let's say it does. His daughter Anne is 1 year old and Louis II hasn't been born yet, while Vladislaus' younger brother Sigismund (future Sigismund I of Poland and Lithuania) was the duke of Glogau and Troppau at the time and was made governor of Silesia and Lower Lusatia that same year. What are the chances of him being elected king of Hungary and Bohemia?
Furthermore, let's assume the succession in Poland and Lithuania follows its OTL trajectory. What is the likelihood of Sigismund becoming the king of Bohemia and Hungary AND Poland and Lithuania? Would the powers that be in any of these Jagiellonian realms be inclined to accept this arrangement? This inheritance also covers an exceedingly vast swathe of Eastern and Central Europe, would its combined might make it easier to keep the Turks at bay or would its unwieldy size distract from resolving Hungary's (or Poland's/Lithuania's/Bohemia's) institutional problems? Feel free to consider any further implications, like the opinion of the Habsburgs and Rurikids on this development or the effect this Jagiellonian mega-union would have on the diffusion of Protestantism.
@Jan Olbracht @Fehérvári @Zygmunt Stary