My opinion is that windows is dying with the advent of Windows 8, the abandonment of XNA, the PR disaster of the Xbox One, and much more. C# and .NET are tied to that sinking ship. Mono has a few key flaws from what I have heard, so I would think about switching languages entirely. I have identified 3 possible routes, but I'm sure there are more:
C++ and Python (industry standard)
Java (almost all platforms, including android and browser)
Html 5 / Javascript (browser)
Do C++. It sucks, it's cumbersome, and it's generally not nice, but there is a lot more code available, and it is industry standard.
If you can't handle C++, Java is pretty similar to C#, and can still make games, and can run in a browser or on android. Unfortunately, there is much less code available in terms of engines and game libraries.
If you are feeling adventurous, HTML 5 is only in browsers, but for a lot of the types of games that are being made a browser is
perfectly fine. Also, lots of developers are active in web-based games, and it has the potential to become a dominant gaming platform. HTML 5 may be a risk in terms of timing in the near future, but long term I expect HTML 5 to be just as widely used as C++.