Analogous to aggregate_fields that you plan to allow developers to define on "array_relationships" .... you might consider providing lookup_fields on "object_relationships".

Lookup fields add the ability to sort and filter on related fields. For example, without lookup_fields the following order_by isn't valid:

{
  work_orders(order_by: contractor_name_asc)  {
    id
    work_order_no
    contractor {
      name
    }
  }
}

End users have a strong expectation when being displayed a data grid / table to be able to click any table header and sort.

After allowing the developer to create a lookup columns "contractor_name" that points to the contractor's name ... the following would be valid:

{
  work_orders(order_by: contractor_name_asc)  {
    id
    work_order_no
    contractor_name
  }
}