Anticipated release: Preview in Q2 2018, with general availability in the second half of the year. Please review the TAP program post, Microsoft is looking for more customers to help validate this release.
Microsoft has not released much information on this release, other than it will focus on security, compliance, usability and manageability. Perhaps there will be some a few leaks in the summer as preview program gets started. Microsoft, however, has confirmed that the timing of the Exchange Server 2019 preview and release milestones will mirror those of SharePoint Server 2019, Office 2019 and Skype for Business Server 2019, but that is all.