The main need I see is for a Reference (in the Diàtaxis sense) about the interaction of tools for building source distributions and wheels with declarative project metadata. Right now, PEP 517’s Build backend interface seems to be the authority on this. PEP 517’s Terminology and goals contributes. Part of what is not clear in PEP 517 is that the scope of “build” is narrow, limited to sdists and wheels. This confused me and some others.
Also, I don’t see a PyPA Specification which really describes the pyproject.toml file. There is a PyPA Declaring build system dependencies specification, but it is a stub which refers out to PEP 518. PEP 518 defines the pyproject.toml file overall, and the [build-system] and [tool] tables therein. PEP 517 defines the build-backend key in the [build-system] table. The Pypa Declaring project metadata specification describes the metadata parts of pyproject.toml well, but not the basic format or the [build-system] and [tool] tables.
I suggest a need for a Reference specifying everything about pyproject.toml files in one place. Maybe the shortest path is to add to the existing Declaring project metadata Specification everything from PEP 518 and PEP 517 about pyproject.toml that is not already there. It could continue with the existing format. The metadata heading would become a section of the overall Reference. It would need a more general title. Alternatively, there could be a new Specification about pyproject.toml overall, and it could delegate the metadata fields specification to the existing document. In either case, the Declaring build system dependencies stub gets replaced by another document.
I don’t see a PyPA Specification which really describes PEP 517’s “format for source trees”. PEP 517 doesn’t seem to describe that format clearly either, except to say it is “a new style of source tree based around the pyproject.toml file”. I suggest a need for a Reference describing the “format for source trees”. Perhaps it would be quite short, just saying, structure it however works for you and your build tools, and put pyproject.toml at the root. Or, maybe we need discussion to crystallise the actual current consensus for required source trees format.