Elector John George I of Saxony played a significant role in the TYW. I note, however that his son was only abt 5yo when it began in 1618, and JG himself had no surviving brothers. I have hunted back via Wiki, but there seem to have been a long string of Electors who were only (surviving) sons of only sons.
Supposing JG had died before the birth of his own son, do we know who would have been the next Elector?