I have come up with something I think I’ll add to the TL, though this may require some retroactive changes.
I have sought for a long time to increase the cultural and linguistic influence of Normandy on the development of France as a nation. After a few false starts, I have come up with an idea I think is worth pursuing and which has a window opened for it in this TL – the Norman conquest of France.
In the 1320s, the king of France names his third son Louis duke of Normandy; Louis is something of a troublemaker and his father, Charles IV, thinks to remove temptation by removing him from Paris. As long as Louis’ eldest brother, Philip V, is on the throne all is well – Philip rules well and he and Louis get on nicely. However, in 1340, shortly after the conquest of Guyenne from Edward II and before John of Chester’s coup, Philip dies suddenly, leaving the throne to the middle brother, Charles.
Charles is dissolute, violent, influenced by his Navarrese wife Elisabeth, not very intelligent and generally a bad king. But his wife, though a foreigner and possibly evil, is very smart and has assembled an inner circle of advisors of great ability if dubious scruples. She has also solidified the support of much of the nobility, though a sizable faction has gone into opposition, siding with the Duke of Normandy.
Louis of Normandy is physically small but powerful with a robust intellect and a strong loyalty to his brother. He hates Elisabeth, however, who reciprocates and leaves Charles in a bind – he loves his brother and wants to think well of him but his wife is constantly poisoning his mind against him until finally a breach is imminent.
Meanwhile, messengers have been sailing back and forth between England and Normandy, holding talks to secure Normandy against English raids should civil war break out. Louis is loyal to the crown, despite his mistreatment, but feels things are going too far in Paris and if action is not taken France herself could be in danger.
In 1343 John II and Louis conduct talks in person on the island of Jersey, itself once a part of Normandy; it is agreed that in the event swords are drawn England will remain neutral, the issue of active English support for Normandy being considered too sensitive to decide until the shit actually hits the fan.
In his years governing Normandy, Louis has all but created an independent kingdom of the Normans, working closely with local governing bodies - the Norman Estates et al. He is very popular, taking a Norman wife, mastering the Norman dialect and conducting business in it; the Normans feel like he is one of them. In 1345 a deputation of town leaders delivers a pledge that whatever happens Normandy will “live and die” with their Duke.
In September 1345, Queen Elisabeth decides Louis has become too powerful; he has several robust heirs while Charles has only one son who, though 15 years old, is not in the best health. She decides to eliminate the threat and persuades her husband Louis is plotting with the English to sever Normandy from France and place it under London’s suzerainty. Her power over her husband is proof against the absurdity of the claim and he summons his brother to Paris to answer the charges. The messenger bearing the summons dislikes the foreign queen and manages to secretly inform Louis of the queen's plot.
Louis responds by stalling for several months, but in the spring of 1346 he has run out of excuses and must simply refuse. He tells his brother plainly that he while he loves him and trusts him implicitly, there are wicked persons at his court who mean to harm the kingdom and the Duke.
By this time, however, Charles is inclined to suspect his own kin and orders him to attend or face the consequences. As the summer of 1346 opens the stage is set for civil war.