WordPress 4.3 lead Konstantin Obenland has created a kickoff post for the release to highlight some areas of focus. The first meeting for the release will be tomorrow, and the target release date is August 18th.
There will be about two months of feature building time, before the feature freeze on July 1st. So while 4.2 just wrapped up, it's time to get involved if you want to help build new features for 4.3.
Feature-wise, six separate admin-centered projects are already slated to better handle small-screen support:
WordPress 4.3 will be all about enabling users of touch and small-screen devices. @ryan [Boren] has been testing flows on a myriad of different devices the past few releases and uncovered many things that desperately need attention.