OKAY I’ve been thinking about this for hours and
I don’t think he’d at all be up to novels. That kind of stick-to-it longhaul commitment is …really not his strength.
I also don’t think he’d go for political commentary; see Not His Strength.
I think, realistically..Grantaire’s writing Urban Fantasy romance short stories. Possibly even illustrating them. or making comics. They’re kinda super dirty and also super sincere under a thin veneer of attempted cynicism.
(This goes DOUBLE in canon era, where he’s totally out there selling painfully heartfelt short stories about strangers meeting at a masquerade ball only one of them turns out to be Spirit of Something and Vanishes At Dawn, having been Too Pure For This Earth. )
He uses a pseudonym and even Bossuet doesn’t know he’s writing these things (and Prouvaire, who does, would never betray a fellow writer’s need for privacy).