Essentially, in 1345 Edward III had his daughter engaged to the King of Castile, but the daughter, Princess Joan, died of the Black Plague before the union could be carried out.
What might have happened had Joan not caught the plague in Bordeaux, or survived?
I believe it will make the Castillan and English relations better than OTL and it might butterfly John of Gaunt's intervention in Portugal and Castile so we might see a United Castile and Portugal unless if the House of Burgundy survives after that POD.
Indeed, I was wondering if it might give Peter a bit more of a, spine, for lack of a better word. And if the connections between England and Spain are strengthened it will likely make the Reformation go differently, if it still occurs.