Okay, so inspired Velasco's TL, I did some research and found out that D. Felipe IV was rather a horndog (he fathered more kids - 30 in total - than either of his nephews - Charles II or Louis XIV, not exactly devoted husbands either.)
These kids started with his first wife, Élisabeth of France in 1621 when he was 16, and ended with the birth of a stillborn daughter by wife no. 2, his niece Mariana of Austria, in 1662 - when he was 67, with those 30 kids - 20 legitimate and the rest bastards in between.
So many bastard sons in fact, that the quip ran at the Spanish court that "the king is a master at producing bastards (sons)".
However, his (legitimate) kids didn't have the greatest start in life - who can blame them, when your dad's your mum's uncle, it gets icky, I also might have just died from embarrassment

.
But that's besides the point. Of those 20 legitimate kids - 4 survived past the age of five and only 2 left issue, and only one still has surviving descendants.
My question is, what if more of Élisabeth's kids - who except for her two sons, Baltasar Carlos (1629-1646) and Fernando Francisco (lived a few hours after birth on 12 March 1633), were another 10 girls - of whom only Maria Teresa (b. 1638) who became queen of France lived - survived infancy?
NOTE: The reason I limit it to Élisabeth is due to the fact that Baltasar Carlos seems to have been far healthier than his half-brothers from his father's second marriage, albeit not as healthy as his illegitimate half-brothers (but that's to be expected).