While still a Prince, Edward took part in the 8th and 9th crusade, during which he was stabbed by an assasian with a supposedly poisoned blade. Edward was weakend and stayed at Acre for many months but eventualy recovered and went on to become Edward I.
However what would have happend had the blade been poisonous enough to kill him? I presume that, given that Henry III is on his way out at this point there would be a regency (led by Robert Burnell and Edmund Plantagenet perhaps?) for King Alphonso I.
I also reckon that, without Edwards force of personality, Llywelyn ap Gruffydd would be able to consolidate and hold onto his principalty of Wales even if Roger Mortimer and the other marcher lords were raiding his terrritory.
What other things would be effected by the death of Edward I before he becomes king?