A newsletter of late 1306, describing the visit of a Spanish cardinal to England, made the astonishing claim that the magnates of Castile had agreed that they would offer the Castilian throne to (the future) Edward II, should his cousin Fernando IV die without a son. And in 1323, Edward tried to claim a share of the county of Provence, as his inheritance from his grandmother Eleanor of Provence. Here's some more information.
As far as I can work out, Edward II is one of only two English monarchs with a Spanish parent, Mary Tudor being the other. In December 1306, a papal nuncio named Pedro, Castilian by birth and cardinal-bishop of Santa Sabina, visited England, and Edward I remarked to him that "he should have a special affection for our dear son Edward, as he [Edward] is of Spanish descent." According to a contemporary newsletter, Pedro had entered into an indenture with the magnates of Castile that Edward, as the son of King Alfonso X’s half-sister Eleanor, would succeed as king of Castile should his cousin Fernando IV die without a male heir.
This story, if true, strikes me as extraordinary; Edward II's mother Eleanor (or Leonor) had seven older half-brothers, and surely there must have been many more candidates for the throne than Edward, through the male line. But there had been a lot of conflict in Castile regarding the succession to the throne. Eleanor's eldest half-brother Alfonso X fathered five sons. The eldest, Fernando de la Cerda ('of the bristle'), predeceased him, leaving two young sons, Alfonso and Fernando. Alfonso X wished his throne to pass to his elder grandson Alfonso, but his second son Sancho demanded that he be made heir to the throne, and precipitated a bloody civil war in 1282. When Alfonso X died in April 1284 - the month his nephew Edward II was born - Sancho seized the throne as Sancho IV. He died in April 1295, leaving a nine-year-old son, Fernando IV. Sancho's brother Juan, the fourth son of Alfonso X, claimed the throne, claiming that his nephew Fernando was illegitimate. The kings of Portugal and Aragon took advantage of the chaos and invaded Castile in 1296, intending to divide the country between them, and the de la Cerda brothers, grandsons of Alfonso X, also continued to claim the throne.
Only Fernando's redoubtable mother Queen Maria de Molina - another close relative of Edward II - saved his throne, and in 1301, when Fernando turned sixteen, the pope finally declared that he was indeed legitimate. Fernando was, however, or at least was perceived by his nobles to be, a weak and ineffectual king, and they were unremittingly hostile to him. He compounded his faults by failing to father a son until he'd been married for ten years.
Given all this, maybe it isn't surprising that the Castilian magnates preferred the thought of the prince of Wales acceding to the throne (assuming the newsletter was correct). Whether Edward would ever have become king of Castile is a fascinating 'what if?', but Fernando IV finally fathered a son, Alfonso XI, in 1311, and thus spared Castile the trauma of being ruled by Edward II.
In February 1323, Edward II suddenly took it into his head to try to claim a share of Provence, and wrote several letters to this effect to Pope John XXII, asking for his help. His grandmother Queen Eleanor was the second of the four daughters of Count Raymond-Berenger V of Provence, while the third sister, Sanchia, married Richard of Cornwall, brother of Edward’s grandfather Henry III; Edward was also her heir. Maybe Edward thought that as the heir of two of the four sisters, he had a good claim. In fact he didn't, as Raymond-Berenger had left the entire county to his fourth daughter Beatrice in his will - to the fury of her sisters, who spent many years asserting their rights to Provence.
Edward also wrote to Beatrice’s grandson Robert, titular king of Jerusalem and Sicily, count of Provence and Edward's second cousin, asking him to "restore to the king amicably" the portions of the county that Edward said fell to him by inheritance. Thomas of Lancaster, another grandson of Eleanor of Provence, had also tried to claim part of the county, and John XXII rebuked him in early 1322 for failing to write courteously enough of Robert of Sicily. Queen Eleanor had transferred her claim to Provence to her Lancaster grandsons Thomas and Henry in May 1286, with reversion to Eleanor’s heirs, i.e. Edward I and Edward II, and Edward confirmed the Lancasters’ rights in the county in June 1319.
Although Edward wrote again to the pope and Robert of Sicily in August 1323, nothing came of it - John XXII politely informed him that he was unable to use his influence with Robert regarding the matter - and he abandoned his efforts. Oh well, it was worth a try, I suppose.
Peter Linehan, ‘The English Mission of Cardinal Petrus Hispanus, the Chronicle of Walter of Guisborough, and news from Castile at Carlisle (1307)’, English Historical Review, 117 (2002), pp. 615-20; Hilda Johnstone, Edward of Carnarvon 1284-1307 (1946), pp. 118-121.
Calendar of Close Rolls 1318-1323, p. 697; Cal Close Rolls 1323-1327, p. 136; Foedera II, i, pp. 396, 507, 531, 534; Calendar of Papal Letters 1305-1342, pp. 447, 455-456; Calendar of Patent Rolls 1281-1292, p. 243; Cal Pat Rolls 1317-1321, p. 341; Nancy Goldstone, Four Queens: The Provençal Sisters Who Ruled Europe (2007), pp. 106-8, 111-12, etc.