Please allow to set self reference to built-in entities, especially Contacts. At this point, built-in entities cannot have self reference (having the record itself in its field).
With Portal Add-on, sometimes customers want to record who had modified the record, like modifiedby field. As Portals treat users as contacts, we have to add custom lookup field for contact to the entity and set the portal user (contact) to the field. But when it comes to Contact entity, it cannot be achieved. If portal user edit their own record (Profile), the operation fails because contact records are not allowed to have self reference.
It's a popular requirement for Portal customers, so please consider to make this possible.