A site builder can design significant parts of the site in the browser
User story
Given that I am logged in as a Site builder,
when I want to change the look, feel, colors, and design of my site
I want to be able to do this visually in my browser so that I'm not required to do the slow work of theming.
Background
There's always been hope that Drutopia, website software as a service, would be the natural home for the designer. Unfortunately, theming is still hard and slow, and most designers are not themers. To truly empower designers, both professionals and people who want to take more control of their own site, we need something that bypasses the theme layer (decoupled Drupal) or goes on top of the theme layer.
Proposed solution
Choose a libre software visual website editor and integrate it with Drutopia (somehow). An initial step may be opening up the landing page, and various regions like header and footer, to editing.
The newest and most promising is probably Tandem. It also hasn't been released yet (as of 2018 February 20) but it's the only one based on web components and meant to be platform agnostic. Their issue queue is open.
Other open source prior art worth looking at is Silex (did not like), BlueGriffon, ApostropheCMS, and concrete5 (some notes on concrete5 here).
And the solution with the most traction in Drupal is WordPress's Gutenberg, see Gutenberg module.
To compare our solution against: The proprietary competition. Wix, Squarespace, Weebly, and perhaps especially Webflow.
Remaining work
So much. But replacing our Paragraphs-based content editing with web components based content editing, and using some of the same approaches and techniques for whole-site editing, would be really awesome.