Oh, and it just hit me, Ferdinand gained in his own right Bohemia ad hunagry by marriage. Now, even without the spansh inheritance, the Jagelliones would enter a marriage pac with Habsburg - both had to fear the Ottoman advance, and anyways, at that time nobody thought the marige pact would immidatly lead to Habsburg inheirting the lands. So Ferdinand would become King of Bohemia and King of Hungary...
I've been reading a bit on this and the arrangements for the marriages were made by Maximilian. Considering he's Emperor, he would still have the power and influence to get these through - after all Hungary wasn't looknig for Spanish involvement, but for Imperial/Austrian involvement
OTL Ferdinand was born in Castile, which would be different here - he would be born in the Netherlands like his brother.
Philip The Handsome OTL died very suddenly in Castile of a chill, or pneumonia induced by a chill, something that is hardly pre-ordained. Had he remained in the Netherlands he may well have lived to outlive his father, and thus succeeded as Emperor in 1519. Had he already been made King of the Romans ? I'm pretty sure of it
Being alive, one assumes that Philip would have the final say in the marriages of his children, although as Emperor and dynastic grand-daddy Maximilian would still have a lot of influence
And of course, as stated before, Charles of Ghent might get married to Mary Tudor, whilst she is still young (which was mooted at the time)
Which all means that whilst this is a Trastamara thread the greatest differences may well come in the Habsburgs...
Looking at the Trastamara, Juan would become King of Castile in 1504 but King of Aragon only on Ferdinand's death in 1516, unless Ferdinand abdicates or decides to let his son be Regent and focus personally on Naples instead. If he does the latter, who knows how butterflies would react to his prospects of a son from Germaine de Foix ?
Its also possible that Upper (Spanish) Navarre would not be annexed if Juan succeeds to the throne of Castile. Ferdinand II only did it in 1512 and the events leading to this could easily be very different
Of course, it would depend on what type of person Juan is - one assumes there is some evidence from his youth for this ?
Best Regards
Grey Wolf