We want to use StructureDefinition as an element library for Questionnaires.  However, for that to work we need all of the extensions that control Questionnaire appearance and behavior to also be declarable on ElementDefinition so that these properties can be set on data elements in 'question libraries' and auto-propagate to questionnaires defined using SDC's modular questionnaire approach.  For example, a re-useable question might want to assert an itemControl extension indicating that checkboxes or multi-select should be used for the question.