Allow users to share private reports with other users

This would be a peer-sharing model, in which users can define access along various lines, such as "allow this group of users to view the report," or "allow this one other user to view AND modify the report." This seems like it would have some value, but also seems potentially quite complex. For example, a CiviCRM report administrator has granted certain access permissions for the report, which permissions are not editable by the user, even on his own private reports; how are those permissions reconciled with the user's own intentions for sharing with other users?

This issue is the place to discuss these considerations.