If archduchess Isabella (daughter of Phillip II) and Archduke Leopold (younger brother of Emperor Rudolph) had had a son, Phillip III would have devolved the Spanish Netherlands to them as a separate country. Phillip hoped that the Dutch Republic would be more willing to reunite with an independent Belgae and end the Dutch Revolt.
If the child was bore between 1605 and 1617, this would have happened during the 12-year Truce of the Dutch War and before the Defenestration of Prague. (And given the ages of both, 1605-1609 is the most likely time of a successful pregnancy).
What are the possible effects of an independent Belgium this early? Parma would still be there, but now in charge on the Belgian army, which would be smaller. the End of the Spanish Road if this becomes a permanent Peace.
Could it short circuit the 30 Years War into just the Bohemian Revolt (1618-20), ending with White Mountain?