I guess this could consist of something like:
A list of proposed properties (for each source)
and mark these as
Required Properties
Suggested (i.e. would be nice to have but not required) Properties
Forbidden Properties
And perhaps for each property a suggested source for it.
We can then solicit feedback by having people 'vote' against the properties with 'required', 'suggested' and 'forbidden'. And of course suggest more properties.