LINQ and ability to interact with any IL compiled language (F#, VB). Also async.
I prefer F# over C#, and I think it is a better competitor to Swift or Java or Kotlin.
The important thing is CLR support. Everything else is just semantics. (literally)
Semantics improves expressiveness which provides conciseness, which leads to less code. Less code is generally less bugs (sure...arguable). But definitely expressiveness is also often correlated to programmer happiness, which is important in itself.
Swift is very similar to C#.
I am just thinking the tooling and the huge .net community who are already building ios with c# in visual studio. If it became core it would be hugely beneficial