Roslyn ships v1.0-rc2 with "Go-Live" license
blogs.msdn.com
blogs.msdn.com
You can design the API as nicely as possible but there is just some information that can't be communicated that way. Now the library can ship with some compile-time code (instead of run-time code) that can analyze your usage of the library and provide feedback.
I'm also curious if C# begins to evolve some meta-programming constructs, letting you write "macros" or syntax extensions in C#. Everyone hates on the C preprocessor (as do I and for good reason) but few compiled languages provide a truly suitable replacement. Back when I was doing C# on a daily basis I would use reflection to generate DynamicMethod instances and emit the IL instructions in 2.0; later being able to convert Expressions to machine code was a step up for the cases where its needed and the T4 template engine works ok for other scenarios.
Now that I'm doing iOS full time that's my one major complaint - complete lack of any meta-programming facility. (That and they took away __conversion)
It's a really nice time to be working as a programmer, although the proliferation of options has gotten to be nearly too much to keep up with even the fraction that I try to.
Of course internal DSLs are limited in their syntax so meta-programming would be welcome. I think it's not the top priority in C# 7 though.
With resharper, I love going through all the little warnings and fix suggestions on old code that people that had written. I think I can have some fun with the analyzers while improving the quality of code.