Microsoft today released Windows 10 SDK Preview Build 17069 to developers. This SDK can be used with Windows 10 Insider Build 17074 or later. At this stage, it, unfortunately, comes without a changelog, but likely includes support for C++/WinRT compilers in the Windows SDK as its headline feature.
The SDK is supported by Visual Studio 2017 and can work side-by-side with older SDKs.
The updated Preview SDK can be downloaded here.