Just an idea: Isabella of Castile dies in 2 October 1470, during the birth of her first child (her daughter also named Isabella). The baby dies too. What happens next? She and Ferdinand had the backing of the nobles who opposed Henry IV of Castile and (later) Juana (called la Beltraneja). Now, without an alternative claimant, would they accept Juana? Or would they support John II of Aragon and Ferdinand as the legitimate Trastamara claimants? What would Ferdinand do?