Here is an unusual PoD:
During the rebellion of 1173 William Marshall reportedly unhorsed the young Richard the Lionheart while covering Henry II's rear guard. After unhorsing Richard, William could have killed the prince, but spared him and killed his horse instead. So what if William had killed the young Richard instead of sparing him? Presumably John becomes king early, after that though, what happens? Can the Angevins even survive more King John? For that matter, is William Marshall still Henry III's regent (assuming of course, there even is a Henry III)? IIRC he was fairly instrumental in saving the Angevins after John.
EDIT: (For that matter, what happens to the Third Crusade?)
During the rebellion of 1173 William Marshall reportedly unhorsed the young Richard the Lionheart while covering Henry II's rear guard. After unhorsing Richard, William could have killed the prince, but spared him and killed his horse instead. So what if William had killed the young Richard instead of sparing him? Presumably John becomes king early, after that though, what happens? Can the Angevins even survive more King John? For that matter, is William Marshall still Henry III's regent (assuming of course, there even is a Henry III)? IIRC he was fairly instrumental in saving the Angevins after John.
EDIT: (For that matter, what happens to the Third Crusade?)