Phase 2 of Gutenberg is all about taking the compelling capabilities of the block editor to the site level. WordPress 5.0 introduced it as the new editor for post content, but that was only the beginning. Blocks are a powerful tool and inspired by the learnings of various other formats of editing that WordPress and other platforms have been using over the years, so it makes sense for them to provide a unified approach to reusable content. The difference between shortcodes and widgets for example has pretty much been non-existent, and it felt weird to create a certain UI component only for one of the two, or having to write some duplicate code to address both layers. With Gutenberg being expanded to become the editor for your entire WordPress site, beyond the post content bubble, this problem will be solved. Furthermore, it will bring the new editor much closer to being able to fully compete with existing layout builders.
Each of the steps to get there (e.g. migrating widgets, menus, and possibly even other elements fully controlled by the theme today) poses several challenges, so it might still be some time until we get to experience all this functionality in its proper implementation. Therefore, inspired by a conversation with Morten at WordCamp Europe a few weeks ago, I wrote a tiny experimental plugin that allows you to use blocks on a sitewide level and explore what the theming of tomorrow could look like already today.