I've modified the DesignerHost-Sample from Microsoft to create a Designer that works in conjunction with Xamarin.Forms, WinForms and GTK#
It's not the swiss army knife and requires a large function that handles the xml-files generated by the designer, but it works!
Basically it loads the xml-files and rescales all sizes of the compontents (i.E. if the form has a size of 320x480 and the device hast 768x1024, the component-size will be changed to width=width2.4 and height=height2.133) the font will also resized. Currently AbsoluteLayout will be used to place the controls "exactly" like the designed form.
Makes it sense to follow it up and is there anyone interested in helping further develop this project?