A supported Windows 10 version (or the next LTSC release of Windows Server) will be required. Another major change is that Office 2019 client apps will not use the traditional installation method, they will use Click-to-Run technology only. Previews of Office 2019 will start shipping in Q2 2018.
Also worth mentioning is that Microsoft is cutting back the extended support of Office 2019 from five to two years. This means users will get a total of seven years of support; five years of mainstream support and two years of extended support.