My plan is to leave it entirely up to the user through Mammoth's style mappings and the users's own css templates.
I'm not really envisioning this plugin being used by user A to convert user B, C, D, and E's docx files automagically. I envision it being used by a writer/user who's adapted a standard for styling all their docx documents. That way, they create a custom Mammoth style-map (or a few) and an associated stylesheet. Once they have that in place, they can focus on creating their Word/LibreOffice documents. The style-map will take care of mapping all standard and custom docx styles/headings to specific html/class-names (with associated css).
In other words ... documents will be created that conform with a pre-existing style-map/css, rather than creating a style-map/css to accommodate each particular document (though the latter is still doable provided the user doesn't mind the extra work).