Functionality from the Beaver Builder team's exceptionally powerful Themer product comes to mind.

With Themer, the entire header, footer, or a "part" area can be built and customized in admin vs. PHP files.

Filters can be applied to use alternate parts on certain pages, replacing the need to add custom PHP files in the WP template hierarchy. Unique and editable page templates can be set up for posts, archives, etc. i.e. single post pages could use a different header than pages with a secondary category navigation.

Very powerful stuff.

I like the comment about how a theme can provide a default view, which an administrator could add to or customize as needed.