Yesterday Matt Mullenweg wrote:
“There are three main focuses this year: the REST API, the editor, and the customizer.
For the REST API we’re going to work on getting first party wp-admin usage of the new endpoints, and hopefully replace all of the core places where we still use admin-ajax.
The editor will endeavour to create a new page and post building experience that makes writing rich posts effortless, and has “blocks” to make it easy what today might take shortcodes, custom HTML, or “mystery meat” embed discovery.
The customizer will help out the editor at first, then shift to bring those fundamental building blocks into something that could allow customization “outside of the box” of post_content, including sidebars and possibly even an entire theme.” (Matt Mullenweg, 2017-01-04)