King Alfonso V of Aragon had no children with his wife, Maria of Castile. Upon his death he was succeeded by younger brother in Aragon and by illegitimate son in Naples. Say that ATL Maria of Castile is not childless-she has 3 surviving kids with Alfonso (Ferdinand b. 1422, Alfonso Jr b. 1426, Eleanor b. 1433). What impact would legitimate offspring of Alfonso have?
I think he would still divide Naples and Aragon-younger son would inherit Naples while older would rule Aragon. That means John, brother of Alfonso V, is only King of Navarre iure uxoris in this TL (and he can't disinherit his son by Blanche in favour of son from second marriage, if he ever remarries ITTL and has any sons by second wife) and would lost even that position after death of Blanche. Thus Kingdom of Navarre could have its own Trastamara line.
Meanwhile daughter of Alfonso V could became Holy Roman Empress instead of her Portuguese cousin and namesake-Alfonso IOTL was in favour of a match between HRE Frederick and his Portuguese niece. If he had his own legitimate daughter it seems obvious he'd preffer said daughter as Empress. For Pope and Emperor match with daughter of King of Naples also should be more desirable than match with Portuguese infanta.