Good news, everyone,
the first draft of the module metadata format is now available for you to comment on. We've decided to go with YAML so it should be fairly readable. You can view the latest version here:
https://pagure.io/fm-metadata/blob/master/f/metadata.yaml
What is is: The file defines basic properties of the module such as its name, version, description, licenses, references to upstream documentation or its content. Currently only RPM content is supported but this can be easily extended in the future. The metadata file is meant as both input and output of the module build process (don't confuse it with package build process), with various tools adding various new data to it, such as vendor and buildsystem identifiers, timestamp of the build, autogenerated lists of licenses or whatever you can think of (well, maybe not whatever but close). The output is then placed in the generated repository, container image or any other module deliverable and can be processed by tools and services consuming and delivering modules.
What is isn't: It's not a SPEC file. It doesn't say how to build individual packages. And it's not a simple comps group either. It can and does provide lots more additional data.
It's not perfect and it's constantly evolving. Please, do comment, ask questions and suggest improvements.
Thanks, P