Thank you Drake. I figured that was one thing that I had bungled up when doing Navatlacas. I suppose that I have to work on that.
My choice would then be between the Dauphin and the Duke of Vendome....
Vendôme would be interesting as King's of France, IMO. Yeah, their the Bourbon line that became Kings of France IOTL... but before Henry IV, they were Catholics. The only reason the Bourbons became Huguenots was because Jeanne of Navarre raised her children as strict Protestants. As far as I can tell though, neither Charles of Vendome nor his son Anthony were inclined towards the Reformation. Anthony converted and abjured his religion so many times like it was going out of style. He would feign Catholicism, convert to the Huguenot faith, then up and abjure
that, claiming he was always a Catholic before
then claiming he was really always a Huguenot.

There's also a funny antecdote that when Jeanne de Navarre allowed the Huguenots to sack the chapel of Vendôme and the churches of the town in 1562, he threatened to send her to a conven. She fled to Béarn. Anthony wasn't
brilliant, but he was totally ambitious, and willing to do anything, especially as he wanted to restore Navarre's borders to what they had been prior to 1512.
So, if the Vendôme branch inherits, they'd probably take Brittany anyways. François, the Dauphin's early death would merely merge it into the crown, and the only other plausible claimant of
Valois blood would be women. Renée of France, the daughter of Louis XII and Anne of Brittany, married to the Duke of Modena. Then the daughters of François Ier and his first wife, Queen Claude (who was the eldest daughter of Louis XII and Anne): Princess Madeleine (the ill-fated first wife of James V), and Margaret, the Duchess of Berry. She remained unmarried until she was 38, when she married the Duke of Savoy.
So, if Charles of Vendôme becomes King, his heir would be the future Antoine, born in 1518. He might initially arrange a match between his son and the Princess Madeleine, to keep Brittany still held close. She's still sickly and dies young, but the Breton Estates agree to accept Antoine as the Duke of Brittany. He later marries Jeanne, and the fun begins when she declares herself a Protestant, banishes Catholicism from her domains in Navarre, and causes shock and scandal in France when they discover their Queen is a heretic.
In typical Antoinean style, he threatens to send her to a convent. It'd be especially hilarious if religious wars still grip France and we see husband and wife on opposite sides of the war. Jeanne, traveling with her army manages to inflict a great victory over the Catholic forces of the Royal Army. Dressed in full plate and astride her horse, she makes a stirring speech denouncing Antoine and lauds herself as a modern day Jeanne de Arc. Incensed, the King of France begins discussions with the Pope regarding the possibility of divorce all while mustering his forces to strike more decisively at the Huguenot rabble...