I assume there's plenty of options... Qt or gtk bindings, electron, etc

Looks like Xamarin is building full cross-platform support via XAML

https://blog.xamarin.com/glimpse-future-xamarin-forms-3-0/

Avalonia seems like a straightforward reimplementation of WPF that is already usable.

https://github.com/AvaloniaUI/Avalonia

DotVVM is working on electron support, and I work with it already. I like it.

https://github.com/riganti/dotvvm-electron

Making it into a .net standard library would make it more usable. Could more easily embed it into other apps, or hook up various front-ends, and they could keep their existing front-end and just have it link to the new library.