Well we can't discuss the Spanish Habsburgs without discussing their inbreeding. Its the reason the line went extinct after all! But I think there are several good PoDs. One would be to have a cadet Branch be successfully established. For instance, if Archdukes Isabella and Albrecht have children then their line could be considered next in line after the main line. Or a better one would be to have Infante-Archduke Carlos, second son of King Philip III, survive. He was only 25 when he died so have him survive and marry, to who IDK, but this line would be the next after Carlos II. That would be an heir right there. Or, and this is my favorite, have Philip IV's son Infante Don Balthasar Carlos, Prince of Asturias survive. No ill health was reported about him, either physical or mental like Carlos II, so he could be a great choice. The main problem the Spanish Habsburgs had was their only sons to survive were the ones they had with their nieces. If the other, less inbreed, sons survive, then its a whole different ballgame.