This is a sub-issue of.
With all page components hopefully converted to blocks (such as page title, primary menus, secondary menus, etc) *and* the proposed possibility to create landing pages with separate block setup / placement / configuration, we need a way to control master layouts where the common elements are placed, so when you create a page with a layout, you don't need to start over and place all the common elements all over again.
Ideally we would have a way to create multiple master layouts that can be used to create pages with different block setup, however, for the pre-feature freeze scope, it seems possible to only support master front end and master backend layouts, where all pages are derived from this layout. Also while we call these layouts on the frontend, the backend implementation calls them displays.
Designs for proposed solution
Listing of two specific layouts possible:
Editing of the layout: