Proposal:

1) Deprecate the builtin simple types ID, IDS, IDREF, IDREFS;

2) Remove all existing co-constraints wrt ID;

3) Remove all the special validation rules and _ad-hoc_ PSVI structures associated with ID/IDREF;

4) Define the semantics of ID/IDREF 'as if' every attr/elt of type ID contributed to a synthetic 'unique' constraint on the validation root of the form
  <xs:select xpath='[all possible parents]'/>
  <xs:field xpath='elt or @attr, as appropriate'/>
and every attr/elt of type IDREF contributed similarly to a synthetic 'keyref' constraint

5) Change the {fields} property of Identity Constraints to be a property record containing {xpath} and a new {distributed} property with a boolean value, mapped from xs:field/@distributed, with semantics of 'false' being _status quo_, and 'true' being, roughly "when this xpath matches something validated as a list, treat each item in that list as a field value"

6) Use that new property to define the semantics of IDS and IDREFS, parallel to (4)