The spec currently doesn't allow you to specify that the "core" optional fields are required. For example, there may be situations where the email address would want to be defined as a required field.
If the fields were marked as required through service definitions, then there would need to be a convention to explain the relationship (redundancy) between those fields and the core "optional" fields.
A clunky way of handling this might be through feedback from an error response if an optional field is left blank yet is needed for some desired function (like emailing the submitter).
Ultimately, it would probably be best to handle "core" fields in the same way as custom fields specified by service definitions.
http://lists.open311.org/groups/discuss/messages/topic/1Yj3jSrYVYjZYuAgpvpGxx