As far as I know Project Reunion is not really a UI framework - it's more about refactoring Windows APIs into individual libraries which can be used across different OS versions. This should enable apps targeting all versions of Windows 10 to use the latest stuff, in a similar fashion to polyfills in browsers.
This is probably to encourage developers to build software using the new Windows 10 APIs. Which is something they definitely have a problem with - I've been using Windows 10 quite heavily since the first beta and I don't think I've ever seen or used a third party Windows 10 (fluent / modern / metro) application.
It is a bit more than that, and I bet with Windows 11 announcement we will get a more clear picture on it.
It is also a way to bring into Win32 side most of the API that really matter on the UWP side of the fence, most likely (although they are yet to confirm this) as migration path to UWP developers.
.NET Native, C++/CX are pretty much dead, and C++/WinRT keeps missing many of the tooling available on C++/CX for XAML applications.
So I expect them to migrate the APIs into an OS version independent layer, and then give the mercy shot on UWP apps.
This is probably to encourage developers to build software using the new Windows 10 APIs. Which is something they definitely have a problem with - I've been using Windows 10 quite heavily since the first beta and I don't think I've ever seen or used a third party Windows 10 (fluent / modern / metro) application.