Admittedly, his death seems to be the easiest to butterfly, since he seems to have died on campaign/in battle.
How's this for a POD then: Martin the Younger pulls through whatever it was that killed him OTL, and succeeds his father as King in Aragon and Sicily on the old man's death in 1410. In 1413, on the death of his father-in-law, his wife, succeeds as Queen Blanca of Navarre. Their marriage has thus far been fruitful:
Miscarried son (1403)
Martin, Principe Ereditario de Sicilia (1406-1407)
Carlos, Principe Ereditario de Sicilia, Duke de Girona, Count of Cervera (b. 1409)
Martin, infante of Aragon and Sicily (b.1411)
Isabel (1413-1415)
Leonor (b.1414)
Isabel (b.1415)
Felipe, infante of Aragon and Sicily (b.1418)
Maria (b.1422)
Crown Prince Carlos, heir to the thrones of Navarre, Aragon and Sicily, is betrothed at first to the French princess, Marie d'Anjou (eldest daughter of Yolande of Aragon, duchesse d'Anjou, daughter of King Martin I's older brother, King Joan I), but as tensions mount between the Aragonese and the Angevin over the childlessness of Queen Giovanna II of Naples (an Anjou), Martin and Blanca start to seek a bride elsewhere.