This feature was first developed as "Engine Extensibility" so the implementation is embodied in an engine service. This makes it next to impossible to use extensibility in other places, like in the console runner, third party runners and even in our own agents.

I propose that we create a separate assembly... NUnit.Extensibility to contain most of the implementation... the existing engine service would simply use an API to access the implementation details.

As a subsquent improvement (but not part of this issue) we could implement the IFrameworkDriverFactory extension in nunit.core using the API directly, without need for full access to engine services.

I'm calling this an idea for now and asking for @nunit/engine-team discussion.