I love this! It will enable so many features in things I’ve built or wanted to build. 
I think it could improve a lot the developer experience for users of libraries that take advantage of this.
I can see several areas where I could use it in features for FastAPI, SQLModel, Typer… even things I haven’t been able to build because there was no way to give a good UX.
It also enables other libraries and ideas I’ve had, e.g. for ML libraries, distributed task systems (like Dask, Ray, Spark).