I've started to work on a Xamarin Forms Design GUI in WPF based on the SharpDevelop WpfDesigner!
If anyone is interested to help, Fork it and submit Pull Requests. There is still a lot of work todo!
I've to create wrappers for all Xamarin Form base Objects, and Designer support for them. If this Base Functionality works, I will style the WPF <Control so they look like the Platform ones and you could switch them so you see how the Controls look on each Platform!
Project can be found at:
https://github.com/jogibear9988/XamarinFormsXamlEditor