Reading this I was thinking that it would be great if we had a similar app focused solely on content creation and editing (no "Discovery" options or changing site-wide options), pretty much like the excellent but discontinued Windows Live Writer.

It could probably take advantage of the WP-Rest API and it would be a tool specifically for Editors and authors who don't (want to) know how to change the site's settings - all they need it to create and publish their content fast and hassle-free (no logins, work offline, manage multiple sites etc).

In fact, if it were open source, developers could modify it to include their custom posts, meta and taxonomies, extend it with plugins (e.g. multilingual or SEO extensions) etc.