The relationships described by the brief vocabulary associated with the MODS element
'relatedItem' is very limited and, as has been pointed out, does not provide the semantics
needed to express the various kinds of relationships that potentially exist among objects.
I would favour an approach that allows external vocabularies to be referenced and the 'type'
attribute to draw its value from the specified vocabulary. An approach might be specified
using xlink, or by introducing new attributes to the 'relatedItem' element. Regardless of
approach, I'd argue that given the broad range of contexts in which MODS is being used,
simply expanding the existing vocabulary is not an adequate approach.