I will allow others to propose a diff, and others can also fork the repository if they want to; nothing is stopping that. (Although, those who wish to fork it must store their own copy (or use chiselapp); I will not host it for them.) If other people want to use GitHub, that is also OK.

Threaded discussions are possible with NNTP. Making a threaded conversation about each line of code (or a group of contiguous lines of code) is also possible; if the message body is formatted in a certain way to find the code in there, then a header could be added to reference the individual line of code (or range) being commented on. (The relevant line(s) of code should also be quoted in the follow-up message.)

However, code patches are not the only kinds of contributions that can be made; there are also documentation, bug reports, feature requests, FAQ, artwork, discussion about how something should be implemented, etc.