Microsoft has announced the next on premises version of SharePoint Server will be called SharePoint 2019 and the preview will be available mid-year in 2018.
Next-Gen Sync Client Support
The Sync Client allows users to synchronize content from SharePoint with their local computers, similar to the tool used for OneDrive for Business; the Sync Client had a checkerd early history but has steadily improved in the Office 365 space. Expect a fast and reliable synchronization experience. It should be noted that Microsoft does not intend to bring support for the Sync Client to SharePoint Server 2016.
Modern UX throughout the product
Modern lists, libraries and pages (let’s hope it means Communication sites, hub sites etc..)
Flow / Power Apps integration
List integration with Flow and PowerApps (more than what we have today). I expect this also means our ability to more easily generate flows to move data from on prem to cloud and vise versa. Microsoft has now confirmed PowerApps and Microsoft Flow as its successor products to InfoPath and SharePoint Designer, used for forms and workflow development respectively, by developers and power users. Interestingly for InfoPath, the form-building tool that's enjoyed a love-hate relationship with SharePoint developers for over a decade, talk of its imminent demise seems premature. Microsoft has flagged its continuation to at least 2026, in line with the expected lifespan of SharePoint Server 2016.
In summary, SharePoint Server 2019 is shaping up as a significant product release. It will continue to support Microsoft customers who wish to maintain the capability for on-premises content storage, while leveraging Microsoft's investments in Office 365 and SharePoint Online, to bring state-of-the-art functionality to SharePoint Server.