Lets say, that quick end came to the House of Trastamara in 1379: in January of that year John only son and heir of king Henry II, dies without issue, followed by father (who IOTL died in May 1379, son's death could hasten it).
What happens then? War of Castilian Succession?
Henry II of Castile still had one legitimate daughter, Eleanor, married to Charles of Navarre, but the throne of Castile is claimed also by monarchs of Portugal and Aragon and by John of Gaunt. Trastamaras ruled Castile for mere decade at this point and legitimacy of their claim was questioned. John of Gaunt would try to exploit situation and launch invasion of Castile few years earlier than IOTL. With what result? IOTL he had Portuguese backing, but in 1379 Portugal had a king, who had his own claim to Castilian throne.
What happens then? War of Castilian Succession?
Henry II of Castile still had one legitimate daughter, Eleanor, married to Charles of Navarre, but the throne of Castile is claimed also by monarchs of Portugal and Aragon and by John of Gaunt. Trastamaras ruled Castile for mere decade at this point and legitimacy of their claim was questioned. John of Gaunt would try to exploit situation and launch invasion of Castile few years earlier than IOTL. With what result? IOTL he had Portuguese backing, but in 1379 Portugal had a king, who had his own claim to Castilian throne.