During the Ninth Crusade, Prince Edward of England, the future Edward I, suffered an attempt on his life. But Edward managed to turn the tables on his would-be assassin and kill him with his own knife.
What if the assassin had successfully killed Edward? What might change?
Their infant Henry would become king, and depending how creative you want to be, he could die on schedule. Eleanor of Castile would immediately rush back to England with Edward to ensure her son's rights, though in OTL, Edmund Crouchback was quite loyal, and Edward actually took his time getting back even in OTL after Henry III died.
There could be a struggle between Eleanor or Edmund as regent, but there was never much OTL documented quarreling between them. Either way, young Henry is tried and kept as healthy as possible, but if young Henry dies, his younger sister Eleanor, followed by the infant Joan are the next two claimants, then followed by Edmund. The assassination would have taken place at least 7 or 8 months before Alfonso would have been conceived.
Well, the Welsh could very well begin to assert further power and independence immediately. If the embers are still there, maybe some Monfortist rebels could decide to come out of the woodwork. Gascony possibly immediately confiscated by Philip III.
Henry IV would need to be immediately betrothed to the daughter of potential prospective ally, all while serious thought is put into the potential husbands of Eleanor and Joan. For the sake of the realm, the English barons and parliament could decide that the crown can only be inherited by male heirs, enabling Edmund, in his late 20s, as the next king. He would be married to Aveline de Forz currently, who died in OTL in 1274. She brought a lot of lands and titles into her marriage, so Edmund would have significant wealth and power. Blanche of Artois may still be the very best candidate, his OTL 2nd wife, and mother of Joan I of Navarre. She did produce 2 sons for Edmund too and was 3 year younger than him. Navarre would be a sensible alliance to make as well, and if Eleanor still has influence, perhaps she could convince Blanche to negotiate with her brother Alfonso X to try and make an alliance and marriage between the newborn Joan and his 3 year old grandson Alfonso de la Cerda.