If Edward, Duke of York, were better known, more people might be aware that he tried to claim the throne of Castile!
Towards the end of Henry IV's reign, Edward found himself part of an expedition led by Thomas, Duke of Clarence (Henry's second son) to Gascony, where the idea was they would aid the Orleanists (or Armagnacs) against the Burgundians in line with current English policy. In the event the two parties in the French civil war had second thoughts, and made a truce with each other. The English were paid to go home - though not wholly in cash. (Hostages were taken and some of these remained in England for a very long time.)
Anyway, instead of going straight home to Philippa, Edward started an intrigue with the King of Aragon and certain discontented Castilians with the intent of making himself King of Castile!
Castile already had a king - Juan I (or John) the son of Edward's double first cousin, Katherine of Lancaster, who was also still very much alive. You may remember that Edmund of Langley and his wife Isabel had long since signed over any claim they had to Castile to John of Gaunt and Constanza, Isabel's elder sister, and thus to Katherine and her children.
Edward's interest in Castile inevitably fizzled away to nothing. John of Gaunt himself had failed in a similar attempt, and he had multiple times Edward's resources. It may be that the Duke of York simply hoped to extract some 'nuisance money' from his Castilian cousins. Whether he would have taken the matter further if he'd managed to survive Agincourt is something we can never know.
However the claim clearly lived on in the heart of the York family as may be seen in the very prominent Castilian heraldry that is displayed in the Edward IV Roll.
The best source for Edward's claim to Castile is the Wylie and Waugh tome. It's rarely mentioned anywhere else.