We have discussed secondary types/mix-ins/aspects/categories/... for CMIS 1.0 and dismissed them to get the specification out of the door.
We should take the discussion up again for CMIS 1.1. It would open the door for more applications and simplify others. If I remember correctly, many repositories have native support for secondary types anyway.

See this document for a more detailed proposal: http://www.oasis-open.org/apps/org/workgroup/cmis/download.php/41368/SecondaryTypesProposal.docx