I suggest you copy pathlib and its tests, and publish it on PyPI with a new name (maybe pathlab.pathlib?). Then, add the new features, tests, documentation, test it in the real world, and after that propose changes into pathlib.
This way, you’re not blocked by anyone until the very end, it can be tested easily, and the improvements will be available even for older versions of Python.
(This is not meant to push you away! It’s a comfortable branch-merge workflow for big changes. Core devs also use similar workflows, see for example importlib-metadata, asyncio/tulip. Or compileall improvements from my colleagues.)