I would suggest just start writing a PEP and post here. People tend to get involved more when there’s something concrete. Don’t worry about the writing too much now; as long as you can lay out your rational, motivation, and a design that makes sense, the rest can be filled out gradually.
See also: How to propose new specs
Edit: Since setup.py syntax is not a standard (it only concerns setuptools), I would suggest starting with extending PEP 508’ extras syntax and describing how to specify a dependency with default extras.