Newly-conquered Roman Gaul is much more interesting than later, more thoroughly Romanised Gaul in my opinion, but that's a pretty awkward fit alongside the more medieval-era stuff that we're also talking about.
I'd be very tempted to push the timeline forward, rather than back, for a setting like this. Not just musketeers, but the beginnings of the Revolution. You can still have the courtly intrigue and glamour (and religious infighting) going on in the circles of the wealthy, but in the more remote and rural regions, or the slums of the cities, things get much more grimy, desperate, and feudal. Not sure if it's quite your thing to build a D&D setting around themes of economic inequality, but if it is, then France would be right up your alley.