Philip II of Spain's first wife during his tenure as Prince of Asturias was, like his father and many Spanish Kings and heirs before him, married to a Portuguese Princess. Her name was Maria Manuela, the daughter of John III of Portugal and the mother of Don Carlos. She was very melancholy, although that's not the topic I wish to discuss today. From her birth in 1527 until 1535, she was also Princess of Portugal in her own right, that is heiress until the birth of her brother.
John III only had two surviving children, Maria Manuela and his son John, the father of Sebastian who predeceased him. Let's assume Prince John is still born, but he dies in 1545 shortly after Maria's marriage to Philip. She does not die giving birth to Don Carlos. Let's give her a long and healthy life, and a little less melancholic. We have to make her seem a suitable successor, after all.
What happens next? The House of Aviz is still far from extinct in 1545; John III's brothers, the Infantes Louis and Henry are still alive, although Louis is unmarried and has only illegitimate children and Henry is a clergymen. Still, both would only be in their forties and Henry especially is far from the farce that involved his succession as an old man in the 1580s. The Duchess of Viseu is also well and alive in 1545, a daughter of Manuel, albeit unmarried, as well Edward, Duke of Guimarãe, a grandson of Manuel. The Duke of Coimbra, George of Lencastre is also kicking around, and despite being an illegitimate son, he is still a son of a King.
Would Maria Manuela be recognized as the heiress of her father, perhaps through the strong arming of the Emperor Charles? Or would the Infantes Louis and Henry press their claims as men being superior over that of their niece, supported by the Cortes? Still, even if they have no children, we would still see an Iberian Union of some sort, although played out in a different way.
An interesting side effect of a surviving Maria Manuela (heiress or not) would also be on poor Mary Tudor down the line. Assuming Edward VI still dies (and let's assume that, because it's fun), there will be no Spanish Prince to capture her attentions and kill her popularity. Might she marry Devon, or might she still look towards her relation the Emperor to provide her with martial advice? Perhaps seeking to secure the Portuguese succession for his son and daughter-in-law, he offers her the hand of the Infante Louis to neutralize a potential rival claimant? England's no Portugal, but a crown's a crown; a Portuguese match, even if supported by Spain, would probably be a little more palpable for the English. Portugal would drag England into no wars, and their friendly relations go back ages.
There would also be the fun idea of the Duchess of Viseu succeeding in some circumstance. Perhaps Portugal's own Virgin Queen, although now I'm just musing.
