Also one of the influences about the union between Aragon and Castille in OTL was the fact that dinasty of Aragon was from 1412 the Trastamara a castillian noble family, so in reality Aragon was governed by castillians (Trastamara) since 1412. A possible POD that will down the probabilities of an union between Castille and Aragon would be that the Caspe Pact of 1412 that conducts to the power to the Trastamara in Aragon had be different that in OTL.
Well, you have another good POD at the Battle of Toro (1472) as other guy says. With a Portuguese/"Juanist" victory over the Aragonese/"Isabelist", many towns and people that IOTL sided with Isabel after the battle (like Madrid, the Duke of Alba or the Master of Calatrava) could join the "Juanist" side and ended the war with a victory for Juana, probably a couple of years earlier and without French involvement. In that case, we have a teen Castilian queen with only 13-15 years, very influenced by his old husband Afonso V of Portugal. In this universe Afonso probably didn't fall in a big clinical depression after losing the war and survive more years than in OTL, may be to see another politic marriage between the Portuguese heir Joao and one child step-sister born after the union of Juana and Afonso (girls are more probable when the father is old

). The final result is a union between Portugal and Castile in wich Lisbon is the master head. The African adventure is the main politic of the kingdom and the dinastic union with Flanders never happens (so, no non-ending Flanders war... and may be the Dutch and English still Catholic!).
OTOH, Aragon would have a very dark future. Without the Castilian proto-tercios, he probably suffers a total defeat in Italy and Naples is united with France. Moreover, the Cerdaña-Roussillon never returns to Aragon and his allies in N-W Italy (Genoa, Milan, ) are invaded by the French. Perhaps in some decades, only the Papal States (pissed of by the French boot) remains as Aragon allies in Italy. At the end of the century, the Aragonese dominions in Malta, Sicily, Sardinia and the Balearic Islands are the main target of Ottoman/Algerian galleys

eek:!!!). And by the 17th-early 18th century, the kingdom remains are divided between Castile-Portugal/Spain and France, like Poland was divided between Austria, Prussia and Russia.