Microsoft announce that Windows Server 2019 is now available in preview with general availability in the second half of 2018, along with System Center 2019. This release will be a Long Term Servicing Channel release and there will be a corresponding Semi Annual Channel release at the same time. Licensing will be the same as for Windows Server 2016 but with a likely increase in CAL pricing.

Find the announcement here: http://bit.ly/2G1rX8B.