The following ValueSet extensions have 1 element, and the element is optional. Update the cardinality on the element in all listed extensions to required: 

https://www.hl7.org/fhir/extension-ordinalvalue.html

https://www.hl7.org/fhir/extension-replaces.html

https://www.hl7.org/fhir/extension-valueset-author.html

https://www.hl7.org/fhir/extension-valueset-activitystatusdate.html

https://www.hl7.org/fhir/extension-valueset-authoritativesource.html

https://www.hl7.org/fhir/extension-valueset-casesensitive.html

https://www.hl7.org/fhir/extension-valueset-concept-comments.html

https://www.hl7.org/fhir/extension-valueset-concept-definition.html

https://www.hl7.org/fhir/extension-valueset-conceptorder.html

https://www.hl7.org/fhir/extension-valueset-deprecated.html

https://www.hl7.org/fhir/extension-valueset-effectivedate.html

https://www.hl7.org/fhir/extension-valueset-expand-group.html   

https://www.hl7.org/fhir/extension-valueset-expand-rules.html

https://www.hl7.org/fhir/extension-valueset-expansionsource.html

https://www.hl7.org/fhir/extension-valueset-expirationdate.html

https://www.hl7.org/fhir/extension-valueset-expression.html

https://www.hl7.org/fhir/extension-valueset-extensible.html   (IMHO this should be removed)

https://www.hl7.org/fhir/extension-valueset-keyword.html

https://www.hl7.org/fhir/extension-valueset-label.html

https://www.hl7.org/fhir/extension-valueset-map.html

https://www.hl7.org/fhir/extension-valueset-othername.html    (note we have another ticket for this extension)

https://www.hl7.org/fhir/extension-valueset-parametersource.html

https://www.hl7.org/fhir/extension-valueset-rules-text.html

https://www.hl7.org/fhir/extension-valueset-sourcereference.html

https://www.hl7.org/fhir/extension-valueset-special-status.html

https://www.hl7.org/fhir/extension-valueset-steward.html

https://www.hl7.org/fhir/extension-valueset-supplement.html

https://www.hl7.org/fhir/extension-valueset-system.html      (I think we have a ticket for this one - its a duplicate)

https://www.hl7.org/fhir/extension-valueset-systemname.html  

https://www.hl7.org/fhir/extension-valueset-systemref.html

https://www.hl7.org/fhir/extension-valueset-toocostly.html

https://www.hl7.org/fhir/extension-valueset-trusted-expansion.html

https://www.hl7.org/fhir/extension-valueset-unclosed.html

https://www.hl7.org/fhir/extension-valueset-usage.html

https://www.hl7.org/fhir/extension-valueset-warning.html

https://www.hl7.org/fhir/extension-valueset-workflowstatus.html