Eleanor of Brittany was an heiress to England, Anjou, Brittany, and Aquitaine and her claim to these were better than that of her uncle, John of England. However, she was imprisoned from 1202.
In 1195, there was a match proposed between Louis VIII of France but the proposal failed due to objections from the Holy Roman Emperor.
What if this marriage had gone ahead? What are the implications and what happens when Louis invades England in 1216?