Eleanor of Aquitaine in our timeline married Louis VII of France, which was a huge deal since it brought what is now southwestern France into France. For a long list of reasons the marriage collapsed, they both secured an annulment, and Eleanor married Henry II of England, ensuring southwestern France was linked to the English crown for three hundred years.
What if the marriage of Eleanor and Louis somehow survived? Say the reconciliation attempt by the Pope actually works and Eleanor then gives birth to a male heir.
Among other things this immediately butterflies away Kings Richard and John of England and all the descendants of Richard, and Philippe Auguste of France and all his descendants. But the Kings of France getting Aquitaine three hundred years earlier is also huge.