From the trends I see, I'd recommend to become a Javascript expert (not just passing knowledge of jQuery).

Then Node.js and Angular. You should be set for the next 10 years.

Outside enterprise, .NET is sinking into irrelevance. I don't know for Xamarin though.

Edit : fwiw my current job is C# / WPF since 2009. So don't take my comment about .NET irrelevance as mindless Microsoft hatred.