The flow for starting a new template from scratch (for a page, home, archive, etc) is a good opportunity to image how theme creation could work from an editor-first perspective. For example, starting a blank template could likely expose a Grid block, where you can draw the main template part areas in a guided way (header, main, footer, sidebar, etc).

Drawing and laying these out would have both achieved an initial layout and established the necessary structure by creating template parts on the fly.

I think this could be initially a bit more exploratory and incorporate some of the work @epiqueras has been doing on #18600.