Talking about inbreeding, could an English/Danish marriage for infante Fernando Alonso be found? if there's a daughter of John and Mary in the appropiate age range, she would be related both to the Trastámara and Habsburgs and the wedding would serve to tighten the relations between Spain and England. As Spain is probably not going to be as involved in continental matters as IOTL, that could be useful.
As
@BlueFlowwer mentioned, Mary and John do have a daughter named Mary, born 1535 who would be the appropriate age for him. Eleanor and João also have a daughter named Beatriz, born in 1530. I imagine she's probably the front runner for Fernando Alonso, but any number of things could frustrate that match: political reasons, haggling over the dowry, she dies as a teenager, the fact that maybe I don't want to see the Spanish Habsburgs and Aviz end up as inbred as they did IOTL...
Aside from those two, the bigger kingdoms are lacking matches: all of François' daughters are too old. He's not yet done having children with Beatriz, but any potential child will be born from ~1540 onward. A Franco-Spanish marriage could help ease tensions, but I can't see Fernando Alonso waiting until the 1550s to get married, it just isn't feasible. The King of Poland's youngest daughter is still four years older than Fernando Alonso. John Zápolya has a bevy of daughters with Anna, Ferdinand's former betrothed, but Hungary's shifting religious outlook / hurt feelings would probably preclude such a match, plus Hungary has very little to offer to Spain.
Ferdinand's brother will have some daughters available: Anne was born in 1533, but again, there's the inbreeding issue and the fact that Charles' other daughters would probably be better used as alliances within the empire; his two eldest daughters will become queens, so the others marrying into Germany doesn't seem too absurd, since they'll likely marry into the Electoral families or larger German principalities like Cleves or Bavaria.
A wild card could be Louis IV of Naples eldest daughter, Francesca. She's a year younger than Queen Mary's daughter. Such a marriage could be useful in mending fences, but would probably require Ferdinand to recognize Louis IV as the legitimate King of Naples. Granted Charles has agreed to this twice (Longwy and Lucca) but that doesn't mean Spain doesn't feel like Naples is still rightfully theirs. But if French control becomes solidified, then Spain would probably be better of recognizing Louis IV and perhaps arranging a marriage between the two royal houses. It could be a helpful tactic to potentially wean Naples away from French influence.
Jesu, Isabella must have a titanium womb for all of them. Poor woman needs a break. Atleast a couple of the sons is most likely gonna enter the church.
Let's just say Ferdinand and Isabella are a better 'match' in that aspect. Ferdinand being around in Spain full time probably helps, too!
Apparently, in this timeline Spanish doctors wash their hands.
More like luck being on Isabella's side. No joke, if she had a serious medical issue during labor (or a serious medical issue at all), I can promise you that the Saint's relics and bones would probably get dragged out before you could even blink an eye.