Twenty Twenty will include full support for the Block Editor, as well as editor styles that make sure the editing experience matches the end result. In order to speed up development, Twenty Twenty will be based on the WordPress theme Chaplin, which was released on the WordPress.org theme directory earlier this summer.
Mockups
You can find full page mockups of Twenty Twenty here:
Timeline
As mentioned in the release plan, WordPress 5.3 is targeted for release on November 12th, 2019. The first release candidate is targeted for October 15th.
Get Involved
If you are interested in contributing to Twenty Twenty, make sure you are following this blog. During the design and development process, there will be weekly meetings starting Monday, September 9, 2019, 2:00 PM CDT in #core-themes.
Theme development will happen on GitHub and in the interest of time, an in-progress version of the theme code has been uploaded here: https://github.com/WordPress/twentytwenty. Once the theme is stable, it will be merged into core and the GitHub repo will be deprecated.
Learn more
If you’re interested in learning more about default themes, you can read the following posts:
