currently, a skin is a jar containing at least META-INF/maven/site.vm, which is the Velocity template file
adding a skin descriptor (as XML with Modello, to be consistent with Maven style) would permit meta-data definition.
The first case I see is Doxia Site Tools prerequisites, to have a skin make clear that it requires a newer version of Doxia Site Tools (which is a dependency of maven-site-plugin = what end-users see): this would permit newer Doxia Site Tools versions to provide new features for skins (like
DOXIASITETOOLS-150) and skins using these features without fearing failing because being used in older maven-site-plugin versions
this would also create a new "skin-model" module that would give us a natural place to document practices around skins