CodeSystem should be a metadata resource, propose adding the following elements:

approvalDate 0..1 date
lastReviewDate 0..1 date
effectivePeriod 0..1 Period
topic 0..* CodeableConcept
author 0..* ContactDetail
editor 0..* ContactDetail
reviewer 0..* ContactDetail
endorser 0..* ContactDetail
relatedArtifact 0..* RelatedArtifact

In addition, deprecate the following extensions:

codesystem-author (covered by the author element above)
codesystem-effectiveDate (covered by the effectivePeriod element above)
codesystem-expirationDate (covered by the effectivePeriod element above)
codesystem-keyWord (covered by the topic element above)