It could be a bit goofy at times, but so can the later versions of Civ in their leader animations. I love Kupe in Civ VI pantomiming that he's going to slit your throat, but it's certainly goofy. Same with Catherine the Great when she's upset with you in Civ IV.This was actually one of my least favourite aspects of Civ3, actually. I found it quite jarring and goofy, visually.
In the end I think leader animations are a matter of personal taste.
Clovis? Charles "The Hammer" Martel? Charlemagne? (the latter possibly as a dual-leader a la Eleanor of Aquitaine in Civ VI, also leading Germany) I don't think you can really go earlier than the Merovingians and have it represent France, or you'd be crossing into the "clump everyone who's lived in the same geographic region into one Civ" representation with the problems that can cause in areas where many different peoples have lived over time.I like this idea as long as it's not set in stone that this leader can only lead for two eras, and not before this era etc.
Regarding France who would be there leader at the beginning of the game if Jeanne d"Arc was only available starting at Medieval. Surely, we wouldn't have a leader from Gaul?
To me there's not a problem with this idea if we could start the game off with someone like Napoleon and then decide to switch to her through something like a government change.