As says the title. WI Isabella of Castille had died in 1470, when she gave birth to the OTL Queen Isabella of Portugal? Assuming the baby also dies the same day, what would happen to Castile?
I looked a bit at genealogy to try to answer the succession question. And I came upon several possible candidates.
First, there is Isabella's husband, Ferdinand II of Aragon. He is a descendant of John I of Castille through his second son (Ferdinand I of Aragon) and that would be sufficient to make him the heir: the bloodline of Henry III of Castille (Ferdinand I's brother) would die out with Isabella I, at least in terms of legitimacy. Ferdinand II being also the King consort of Isabella, I think this increase his chances. He probably is the one in better position to claim the crown.
A second possibility would be Henry IV's daughter: Joanna la Beltraneja. However, she was suspected of illegitimacy as Henry IV was said to be impotent while her mother had an affair with a Castilian noble. Henry IV did try to make her heiress of Castile but she lost her struggle against Isabella I: anti-Ferdinand noble might raise her standard but I'm not sure she will be able to claim the Castilian crown.
A last candidate would be a cousin of Ferdinand II of Aragon: Ferdinand I of Naples, the illegitimate son of Alphonso V of Aragon. Alphonso V was the eldest brother of John II of Aragon (Ferdinand II's father), so Ferdinand I of Naples is genealogically senior to Ferdinand II of Aragon. However, he is a bastard son and rules the Italian Kingdom of Naples: not sure he will wish nor will want to fight Ferdinand II over the Castilian crown.
We have a wild card in the scenario though: Louis XI of France. Aragon is not an ally of the Spider King and I'm sure he will at least try to be a nuisance to Ferdinand II. It's possible Ferdinand will seek an ally against the Spider King via remarriage or seek alliances. A natural ally would be Charles the Bold, Duke of Burgundy and arch-ennemy of the French King. Another place to look is Yorkist England: though Louis XI did his best to play neutral (supporting whoever could be advantageous for him), the Yorkist had generally non-French policies: plus, Louis XI supported the short-lived Lancastrian restoration of 1470-71, so Edward IV could negotiate with Louis' ennemies.