[terrajobst]

In a sense, that's what we have been doing. But the challenge is that .NET Core is an actual implementation and the other .NET stacks have their own (i.e. the code base isn't shared, Mono/Xamarin/Unity are on a different train than .NET Framework/.NET Core).

For newer .NET implementations we push folks to start with .NET Core and adding their specific technologies on top. That is, for instance, what Samsung has been doing with Tizen. So if the open source community innovates in .NET Core, Samsung can just move to a later build of .NET Core in order to benefit from it. They are a pure superset of .NET Core, by construction. However, for all other cases someone needs to port the changes from .NET Core to Mono which makes it consumable by Xamarin and Unity. .NET Framework is in a similar boat, although porting is somewhat easier as the .NET Core code base originated from .NET Framework.

Since we can't (easily) move Mono/Xamarin/Unity/.NET Framework on top of .NET Core, we need a way to standardize the API set so that it's not all chaos. And that's where .NET Standard comes in.