There are a lot of goodies that are seriously buried down in private and internal classes and methods.
One example is Device.PlatformServices - I don't see anything there that should be internal but lots of good stuff.
internal interface IPlatformServices
Same goes for using Reflection - there are interesting tricks used all over the Xamarin.Forms library, it would have been nice to expose them to the rest of the world as well (see ImageSource.FromXYZ)
Assembly assembly = (Assembly)typeof(Assembly).GetTypeInfo ().GetDeclaredMethod ("GetEntryAssembly").Invoke (null, new object[0]);
Another example is access to underlying data obtained by ImageSource.FromXYZ - I want to be able to edit the stream and load it back in. It's possible if I use ImageSource.FromStream explicitly but if I use ImageSource.FromUri it's not accessible. Neither is the cache.