You can do this now in Webflow using Firebase as a backend.... just a thought... so it is already possible to do this editing of webflow content inline, on a site hosted on webflow server.
Another possibility, if you want to host the sites you make with webflow on your own servers or your clients own servers, is to use a service like Surreal CMS or Pagelime CMS, which let you assign class names to items to make them editable, and then your clients can log into the online CMS of choice, and edit pages that way too.
I prefer Firebase, so that sites can be still hosted with webflow, and not have to deal with moving sites around to different servers etc. Regarding Sketch, it is a great tool. I agree with you, webflow is the best tool out there and most fun to use