I rewrote the PEP based on the latest iteration of the design concept: https://github.com/python/peps/commit/9a8e590a523bdeed0598084a0782fb07fc15dfc0

(That initial commit has some minor errors in it that I've already fixed, so check the latest version in the repo if you spot anything else - if I'd thought it through properly, I would have submitted the update as a PR, so folks had a place to more easily make line-by-line comments)

I genuinely like this version, and I think it should be reasonably straightforward to implement (given that types.MappingProxyType already exists).