But a plugin generator of mansards would result superb.

I believe it can already do that. Of course, at the moment, it needs more than just a piece of roof.
To do this, I first need to make a plan in SH3D. Reproducing this project involves a lot of work.
Since it's not my project and you've already done the facades (which look great), I suggested you share the .sh3d file and I could try to do the roof for you.
Your work in FreeCAD look good but my intention is to make as much as possible in SH3D.

I suspect that at some point this type of roof will be integrated in the Roof Generator plugin.

I working on this but I don't know when/if will be something usable.

PS: On this architecture style the "ornaments" have also an practical function not only to look beauty. In this case, at roof it mask the rain gutters.
At facade it mask different walls thicknesses between levels.