I have an idea to develop: Étienne Marcel (
https://en.m.wikipedia.org/wiki/Étienne_Marcel) was a wealth Paris merchant, Provost of Paris Merchants (a sort of Mayor), who led the Third Estate in the General Estates during the Hundred Years’ War. When aristocracy, with Royal support, tried to stop his reform movement in 1358 he revolted and killed aristocrats representatives before the Dauphin. Then he tried to ally with the Jacqueries and Charles the Bad of Navarre to defeat his enemies but, fearing he would ally with the hated English Crown, he was betrayed and killed by a mob.
Now let suppose Marcel is quicker and smarter: during the assault to the Royal Palace he take the Dauphin Charles as hostage. With King John II prisoner in England after Poitiers, he found himself at the helm of France. He forced Charles to approve all conditions asked and with these builds up his support among merchant-urban class and poors. Loyal aristocrats are able to make Charles escape and gather an army, so Marcel decide to eliminate the cause of the problem, the Monarch himself. As Charles of Navarre or the English King give not more assurance then Dauphin Charles regarding Marcel and merchants’ rights, Marcel strike an alliance with the Jacqueries and declares the Monarchy abolished in favor of a Venetia/Florence-style oligarchic republic. Thanks to merchants wealthy he will be able to quickly assemble an army to counter Valois, Plantagenet and Navarre restoration attempts: when the Dauphin attacks he is taken by surprise and defeated, dying in the battle. With Charles dead for 1360 and John II prisoner, the next in line is Louis, Duke of Anjou, then John, Duke of Berry, and Philip the Bold, future Duke of Burgundy, but all three were prisoner in England, so the royal cause is left without a leader. Thanks that the Republic will be able to survive attacks, at least until Joan of Arc will came to save France from its enemies.