One thing I'd like to look into is implementing a series of features that should help users become contributors. I thought that a good place where to include the extra information would be AppStream

What I suggest is to extend the <url/> tag with two aditional types: contribute and vcs (i.e. version control system, a different tag could help. contribute would link to a main contribute page if available and vcs would actually link to the git repository (or similar).

For example, for appstream itself it would be:

  <url type="homepage">https://www.freedesktop.org/wiki/Distributions/AppStream/</url>
  <url type="vcs">https://github.com/ximion/appstream.git</url>
  <url type="contribute">https://github.com/ximion/appstream/</url>

The end goal is that the system should have all the information required to lead the user from an application to its sources and the community that manages them.