One could say that Elizabeth marriage to Albert was the culmination of a series of agreements between the Luxemburgs and the Habsburgs that goes since Charles IV, that regarded the House of Austria as the natural heirs to the Imperial Crown in case his offspring failed. So for your challenge to succeed you have to either kill all the Habsburgs (unlikely, at the earlier 15th century there were too many of them) or cause a fallout between the two dynasties, leading Siegmund to find another heir.
The most natural candidate might look like
Wladyslaw (Jogalia) II of Poland, but though he looks powerful and stuff, maybe even accepted by the Hungarians, there is no way the Prince-electors are gonna accept him as their King/Emperor, which was Siegmund's goal, to be heir of his it has to be heir to the Crown of Charlemagne, which Albert briefly did. It restricts our choice to dynasties inside the Empire, since the Habsburgs are out, there is the Wittelbachs, heirs to HRE Louis IV that put them in the same league as the Austrians, that were a lot of them at the time, I don't know who is the eldest there, maybe the Elector Palatine. Other than them the only ones I would consider would be the Wettins.
EDIT: For the impact, the very nature of the HRE, the Hussite Revolution and the Ottomans makes whoever succeed the Luxemburgs to have a bomb on hands, of course the Habsburgs overall failure to hold all those titles sorted (mostly) the things, the Habsburgs managed, barely, the politics of the HRE, Mathias Corvinus stalled the Ottomans, the Bohemians... messed stuff; so if Elizabeth's ATL husband is competent he may survive his father-in-law's legacy in some shape (don't expect any magical king sorting everything).