(...) I'd still want to see contribution structured around a mailing list. Strongly encourage talking about code/ideas before submission, but most people will not read it and will submit directly anyway.

Django enforces this strongly. Anything other than trivial pull requests will be ignored. Contributors are strongly advices to start a discussion on the dev mailing list before working on code and for anything big there's now a formal process for 'Django Enhancement Proposals' (obviously modelled on Python's PEPs)