One of the advanced (or developer) features that i'd love to see is an import tool for CSS Libraries. I'd like to see where we could upload a CSS file, and it'd be automatically parsed and added into the Designer's CSS listing. Then, anyone used to working with a particular CSS library wouldn't have to recreate it when coming to Webflow and the powerful Webflow Style Manager could be used to see what's in use and what isn't.
Although, with this said, I would suggest that imported CSS libraries be exempt from "style cleanup" as they all ought to stay present, even if unused at the current moment... except potentially at the moment of export.
Import Framework CSS/SCSS library like Foundation 6 Merged
Ability to Import a CSS style sheet Merged