Probably a delayed War of Spanish Succession... Unless the child of Charles II of Spain is declared unfit to rule. There are several known cases of princes that were taken out of the succession because of their mental condition.
But honestly, I'm not even sure that scenario is possible... For one, I believe there are proof that Charles II was infertile thanks to a recent genetics study. Second, given how inbred Charles II was and how the child would still be (both of Charles II's wife were related to him if I'm not wrong: I'm sure about the first (a daughter of the Duke of Orléans, which made her a granddaughter of Anne of Austria and thus a descendant of one of Charles' aunt) but I have doubts about the second), I'm not sure he would be viable. Third, Charles II already had a lot of health problems: I don't want to imagine how is son would look if he is WORSE. This also shortens his life expectancy which is already not very high (considering Charles II himself died at 39).