Say Vladislaus II of Bohemia and Hungary is even less fortunate with his offspring than IOTL-his daughter Anna dies aged five and Louis is not only mentally retarded but also of poor physical health and dies in summer 1513. Due to grief king dies the same year, in September.
What does it mean? After death of Anna Janos Zápolya can't hope to marry her, thus it is likely he would marry Margaret Hohenzollern (born 1483), niece of Vladislaus proposed to him IOTL in 1509. So Zápolya would get family connection to Jagiellons, strenghtened later by marriage of his sister Barbara to Sigismund of Poland. At the time of ATL Vladislaus death Sigismund was still anti-Habsburg due to Emperor's support to Teutonic Order. Habsburg-Muscovite alliance from 1514 and loss of Smolensk made him more defetist, as result he decided to make agreement with Habsburgs. But in 1513 he will fully support Zapolya's party against Habsburgs.
So Vladislaus II is dead and leaves no surviving kids, Janos Zápolya who is wealthliest magnate in Hungary, is married to king's niece and has support of middle nobility claims the throne. Emperor Maximilian claims thrones of Hungary and Bohemia for himself (his grandsons are only 13 and 10) but War of the League of Cambrai is not over, so his attention would be distracted, Sigismund support his brother-in-law' struggle for Hungarian crown and propably claims Czech throne for himself. Maximilian would look for alliances with Teutonic Knights and Muscovy in hurry to distract Poles.
Who is going to win Vladislaus' thrones? Is Zápolya able to estabilish his family as new royal house of Hungary in such circumstances?