HTML5.NET could be a real target platform. Multi-selectable for PCLs. You would use the MVVM pattern to share code across multiple native platforms (iOS, Android, WinStore, etc) and HTML5. Just like Xamarin native, but for the web as well.
Visit the HTML5.NET website for more details.
UPDATE: This project should actually be called MonoHTML5! And there is interesting overlap with Unity’s IL2PP project.
Visit the WearableC64 website for more details.
The MonoTizen project aims to build a fully-fledged .NET development environment for Tizen using the Mono Runtime.
Mono is an open source, cross-platform, implementation of C# and the CLR that is binary compatible with Microsoft.NET.
Tizen is an open source, standards-based software platform supported by leading mobile operators, device manufacturers, and silicon suppliers for multiple device categories such as smartphones, tablets, netbooks, in-vehicle infotainment devices, and smart TVs.
Visit the MonoTizen website for more details.