During the Ignite 2015 Conference it has been announced that there will be no SharePoint Designer 2016.
Will there be another tool provided?
How should I manage and maintain workflow, content type, list and so on - without SPD?
Since SharePoint Server 2016 is built on the same code base as SharePoint 2013 and the fact that you still can use SharePoint Designer 2013 on SharePoint Online, there are reasons to believe that SharePoint Designer 2013 will function very well with SharePoint Server 2016. There may be features that can’t be used, but you have other tools available such as Visual Studio, PowerShell, Browser and Design Manager.
You can still use SharePoint Designer 2013 64x with SharePoint Server 2016.