When designing architecture of the dialogs, our main priority was to be absolutely consistent with the Microsoft dialogs, not only functional but also visual. For that reason will be difficult to add such style view for selecting files and folders. We will consider your proffer but I can not guarantee that will be approved. My advice is to try to achieve the desired behavior through modifying the template.
About your second question, can you confirm that this feature request
is what you want?
Want to extend the target reach of your WPF applications, leveraging iOS, Android, and UWP? Try UI for Xamarin
, a suite of polished and feature-rich components for the Xamarin framework, which allow
you to write beautiful native mobile apps using a single shared C# codebase.