OTL Enrique of Aragon, duke of Segorbe and paternal cousin to Fernando, projected to marry Juana c. 1472 but as his cousin had already married Isabel in 1469, he was not supported by his uncle King John II. If Fernando is eliminated, either by the survival of his elder brother Charles of Viana or by his own death, an Enrique-Juana ticket is still possible. In 1492, you would have in Castilla Enrique and Juana and in Aragon Charles and Catarina of Portugal. There is a crackpot theory that Colombus was actually the illegitimate son of Charles, but I do not buy it. On the other hand, one of the early supporters of Columbus was Luis de la Cerda, Duke (well it was an Isabellian title, so he might be still Count ITTL) of Medinaceli, and husband of Charles' illegitimate daughter, Leonor. So there was a connection between Colombus and King Charles (even if Leonor died 1477). If you want to spice it up a little more, you can have the Basque ports given to Navarra (of which Charles was also King) in exchange for Charles' support of Enrique and Juana in the Castillan Civil War. So there is both a connection and a departure harbor.