IOTL, John de Montfort, half-brother of John III, Duke of Brittany, was a contender for the Ducal title against his half-niece, Joanna, and her husband, Charles of Blois, as part of the Hundred Years War. He married Joanna, brother of Louis I, Count of Flanders, and aunt of Louis II, Count of Flanders, both of whom were inclined towards the French. John and Joanna's son, John IV (V to the English), would later be recognised as the Duke of Brittany.
Anyway, your challenge, if you choose to accept it, is to make it so that John de Montfort becomes Count of Flanders by right of his wife instead of/as well as Duke of Brittany. Personally, the best way I could see it is if Charles of Blois wins the Breton succession war, Louis I/Louis II of Flanders are unable to take back/return to Flanders and the Flemish nobles choose to recognise Joanna of Flanders as Countess instead.
Any thoughts?