Microsoft has opened sourced the C# compiler and dotnetcore.
That ought to mean that if I fork the repos, I can compile the eco system and start making changes.
Create C##.
C## is just a silly name, if the trademark is the problem then lets call it FISH.
Would it be legal under the licence that Microsoft have on the source to then share my C## with the world?
There are a lot of reasons why this would be dumb, but I want to focus on the legal aspect of it. How libre the license is.
Here is the license for the C# compiler (Roslyn): https://github.com/dotnet/roslyn/blob/master/License.txt
The license is an MIT which means that you should be able to fork the language and do anything you want with it.
The names "C#" and ".NET" are Microsoft trademarks and so you would need to use some other name