First, I am really happy that you are interested in this and that your point (2) of what you want to see done is very limited and acknowledges that it isn't going to specify everything!  Because that isn't possible. :)

Unfortunately I feel that adding syntax like this to the language itself is not useful without enforcement because it that leads to code being written with unintentionally incorrect annotations that winds up deployed in libraries that later become a problem as soon as an actual analysis tool attempts to run over something that uses that unknowingly incorrectly specified code in a place where it cannot be easily updated (like the standard library).

At the summit in Montreal earlier this year Łukasz Langa (cc'd) volunteered to lead writing the PEP on Python type hinting based on the many existing implementations of such things (including mypy, cython, numba and pytypedecl). I believe he has an initial draft he intends to send out soon. I'll let him speak to that.

Looks like Łukasz already responded, I'll stop writing now and go read that. :)

Personal opinion from experience trying: You can't express the depth of types for an interface within the Python language syntax itself (assuming hacks such as specially formatted comments, strings or docstrings do not count). Forward references to things that haven't even been defined yet are common. You often want an ability to specify a duck type interface rather than a specific type.  I think he has those points covered better than I do.