The last Franconian duke of Prussia had two sons, Albrecht Friedrich (b.1580, d.1580) and Wilhelm Friedrich (b.1585, d.1586). However, both died in infancy and when the loopy duke died in 1618 (having been insane since 1578), the duchy passed to the electors of Brandenburg.
So, WI: the Franconian line in Prussia DOESN'T die out in the male line. I can't find what killed his two sons, but both died at around 6 months of age. What would this mean for Brandenburg if it doesn't inherit Prussia? Or would we see a sort of Habsburg-style internarriage between the branches in Berlin and Prussia?