As far as I know WPF has not been open sourced yet. And even if it was, the framework relies on DirectX, a Linux/Mac/mobile port would surely be problematic.
Right now, .net Core seems to be made solely for ASP.NET . Which is great and a good place to start, but still, I would like to develop cross platform client-side GUIs. The .net foundation has a quite prominently placed link to the MVVM Light Toolkit http://www.dotnetfoundation.org/mvvm-light-toolkit . It is described as a cross platform toolkit that allows the developer to create GUI's for WPF, Silverlight, Windows RT, ... sounds great. But honestly, I don't understand what this is supposed to do. If it was just an alternative to WPF, then Microsoft wouldn't advertise it. If it runs cross platform, why emphasize WPF, which hasn't been ported yet.
The website of MVVM Light itself http://www.mvvmlight.net/ reveals that this project relies heaviliy on Mono and Xamarin. Both of which are not entirely open source. A Xamarin license costs dearly. Microsoft announced that in the future it would support Android and iOs next to Windows RT. Fantastic ! But how ? Hopefully not by redirecting us to Xamarin.
Does MVVM run on .net core ? Is it supposed to run on .net core in the near future ? Is there a cross platform GUI framework that's already usable or in active development ?