For what it's worth, I (representing FastAPI) think this is a great idea. 🎉
More details about my support for this are below (feel free to skip them):
Even if the proposed change requires modifications (or not), the current state is already very helpful, and it would be a huge improvement in the development experience using FastAPI/pydantic and other similar libraries.
By being part of standard Python it would be more straightforward for different editors to implement support for this in a standard way. And by having this simple way to declare that "dataclass-like" behavior, more libraries could take advantage of these features from type annotations similar to dataclasses, and much better support from editors (constructor autocompletion and type checks).
I could imagine several libraries that would benefit from this, even some that are way older than dataclasses, like SQLAlchemy, or also other future possible tools.
The benefits are immediately evident and would help a lot of users (see that PR for a screenshot of autocompletion in a constructor with type info).
All the best,