I think it would be awesome to have the ability to build a style guide in webflow before building your site. Setup your colors, headers, font styling, typography, and more, then once your elements are in place you can begin building your site from a set of predefined elements and classes. The ability to import and export these at will would also be great. Something along the lines of this:
This gives you the ability to ideate without having to commit anything to page/code upfront and would go a long toward helping project organization and structure in webflow. It would also further differentiate Webflow from it's competitors.