While WordPress 4.8 focused on adding new widgets, visual improvements to links in the text editor, and a new dashboard widget that displays nearby events, WordPress 4.9 places a heavy emphasis on customization.
In WordPress 4.9, the Customizer has a new publish button with options to publish, save draft, or schedule changes. Edits made via the Customizer are called changesets that have status’ similar to posts. These improvements were incorporated from the Customize Snapshots and Customize Posts feature plugins.

Those who design sites will appreciate the ability to easily share a link that provides a front-end preview to changes. Note the About This Site widget at the bottom of the page.
This eliminates the need to publish changes to a live site or give users access to the WordPress backend. Links are generated by saving a draft in the Customizer.
Clicking the Discharge Changes link removes unpublished edits. Scheduling changes is as simple as choosing a day and time for them to take place.
These are just a few of the improvements in WordPress 4.9 which you can try out for yourself by downloading and testing WordPress 4.9 beta 1 on a test site. Alternatively, you can install the WordPress Beta Testing plugin on a test site, configure it for point release nightlies, and update to 4.9 Beta 1.
Stay tuned as we go in-depth on some of the other features in WordPress 4.9 in the coming days.
I see a lot of changes being made that are specific to user experience. Like scheduling changes, preview links, and improved theme browsing in the customizer. Tells a lot about where WordPress is heading.