Thanks for disagreeing with me! You're much more of an expert than I am....
Thanks, but full stop there. I'm not an expert on these things (let's say an enlightened enthusiast at very best), and even if I was, I don't get to "win" threads merely by posting on it.
You have opinions and arguments that I can respect or understand even if I disagree with, as long we can discuss it on actual evidence avaible without any self-appeal to authority (which would be, frankly, ridiculous).
Anyway.
I was thinking of huguenot=french protestant (and usobg a wide definition of protestant) rather than huguenot= french Calvinist
The latter being essentially equivalent, tough. Lutheranism had a limited appeal on France, more as an influential feature within french Catholicism (with gallican overtones and not that distinguished from humanism) than an outright movement of its own.
So I agree that my scenario is nor very plausible but I will hold out that it is still possible....
Well, it is possible, but as you said, would imply such a radicalization that a Protestant king of France would probably be dominating only part of the kingdom, probably against a Catholic claimant (Bourbon, or otherwise related to Capetian main line, and you'd always find one) during a TYW equivalent, which would probably make Hapsburg gaining the upper hand.
That said, you could end, by some eventuality that I won't discuss there to keep it open, having a Catholic king of France reigning over Protestant/Protestantized entities. Such as
Provinces de l'Union : roughly, Protestant assembly in Languedoc declared an autonomy of the province, with Henri de Condé as lieutenant, while still abiding by Valois' authority. It was short-lived IOTL, but with a French War of Religion going really worse, maybe it could evolve into something more stable.
If you couple that with a maintain of a Valois dynasty, but a weakened one after ATL Religious War, which could imply a nobility and parlementarian elite calling for regular Estates Generals as the rule and no longer the exception; and an increase of Parlementarian and Provincial Estates power...
Rather than a Protestant dominated France, you'd have Protestant-dominated (politically, rather than numerically) provinces and assemblies within the kingdom. How long could it last is anyone's guess (my own is, not that long) but it could lead to an interesting ATL development of modern and late modern French royal state.
Thinking about it, it could be a stepstone for a Gallican, half-Royal/half-Catholic/half-Protestant (yes, you read that right) mindset; or just a respise before Royal power in France recovers and pulls a "cujus regio, ejus religio ".