In reply to David McCan.

Would it make sense to have the ability to create post type templates first? For that we would need placeholders for title, featured image, content, post meta fields, comments, tax terms, related posts, read more, and navigation blocks for singe and archive.

I don't think it matters if the CPT/user-created templates exist first. As far as the underlying code for that goes, it's super simple to add a check for the theme-based templates at the same time.

But, we definitely need many, many more placeholder-type blocks that essentially replace template tags.

Perhaps the Customizer would be a place where you have a page or set of pages with content like the theme unit test data where you could set global style options for buttons, tables, quotes, etc.? Or override the theme’s defaults.

From what I'm seeing in this ticket, it will be a part of the block system rather than the customizer. It would be nice to see a comparison between the two before any final decisions are made.

Will block-based theme’s have hooks for replacing the theme’s default header and footer? Or will the theme have block areas like we currently have widget areas?

That's an interesting question, and I think it's one of those that does not yet have an answer.