I am hoping this is a joint cooperation with Gutenberg and Automattic. Developing full site editing can at the same time also have a very good influence on the development of Gutenberg. As new specific features might need to be focused on now rather then later.

Here is an Gutenberg issue from over a year ago in regards to:
“Starting out blank or with selecting a template layout.”
https://github.com/WordPress/gutenberg/issues/5537

As it shows some initial thoughts on how to begin with a preexisting page layout. It would be great to continue this exploration with the Automattic team who are working on the full site editing experience. There will naturally be many overlapping features that should be worked on as a joint project.

It would be good to have a discussion about this project during one of the core-editor open office Slack meetings.