Rather than omitting the ns entirely, or trying to list every affected namespace, it might be preferable to have the ChangeStreamEventTransformation synthesize a namespace for the EOT based on the stream type.

A single-coll stream would have ns: {db: db, coll: coll}.

{}A database stream would have ns: {db: db}

{}A whole-cluster stream would have ns: {db: admin}.

{}The latter two potentially also including coll: "$cmd".