My only problem with this is that I'm not sure if Juan - if he does survive - could get Catherine. I'll probably have the civil war that ensues with Francis Phoebus's death still happen, and then Madeleine of Valois is always going to pick a French husband - i.e. John of Albret - over a Spanish one. I know there were plans for Juan to marry Edward IV's daughter Katherine OTL, with an agreement being made in 1479 and ambassadors being sent to Spain to get the details sorted in 1482, and obviously Juan did marry Margaret of Austria just before he died. So a daughter of either Edward IV or Maximilian seems to me to be the most likely match for any surviving Juan (again, if that happens...). I'm not against a Catherine of Navarre marriage either, per se - I'm just not sure if it would be likely.