Mandatory elements (slide 12+13):

Currently the listMandatoryElements operation returns a UsageRecord document
with empty elements as placeholder for the required ones. The proposal is to
replace this return type with a list of QNames of the required elements.

Instead of only returning a list of QNames, it would be interesting to also
allow restrictions on the value of elements.

One idea would be to return a XPath expression that needs to match for any
UR stored in the RUS. This would provide great flexibility.

If different constrains apply for different roles/users the operations may
return different results depending on the role/id of the requester. 
This could also with advantage be realized using different endpoints to the
same service.