C# is a widely used programming language, so having support fot it will expand the guix potential userbase by a lot.

AFAIK the only things needed for this are:

https://github.com/dotnet/runtime to run .NET managed applications
https://github.com/dotnet/sdk To build apps

Just in case this is helpful, there is also a source-build https://github.com/dotnet/source-build/