<Do you think that it would be good to be able to give a "fill" property on closed curves to define that kind of 2D stuff? Or would it be good to just focus on doing it on surfaces only?>

Yes, just a fill property on closed curves as 2D stuff. Anyway I didnt think about folding and bendig that means proplems, yes. But only for a 2D part would be proper solution for sections and layout plans. But! Maybe there are some issue with styles in this case. If I define a style for a "plane" and then define a "fill" as well finally I have got 2 kind of style...? Or "fill" woud be equal with "a type of style" ?
I think that a multiple or "sub"style system would be better because if in the future the section "script" or "command" will be available the automatic generator get the data from "style" system and when it generate fills from the cutted elements it get the "fill type" from the secondary slot.