One of the constraints we have is that any code you write on Linux, we want it to work on Darwin and any other platform Foundation compiles on, and vice versa. I know for a fact there’s a lot of value in this; it does mean we need to coordinate between releases we can’t talk much about and the open source library as it is.
My hope is to be able to share more about goals soon, but, for example, Swift 4.2 has had some effort in bringing functionality to unimplemented parts of the library (with a deeper focus: e.g. bridging now works on Linux, removing a big syntax discrepancy between platforms).