Federation is a fantastic capability. One way it could be improved would be to provide the ability to assign mixin node types as part of federation. Currently, any mixin to be assigned must be assigned after the node appears as visible to JCR, via the Java API. For some use cases, it would be very helpful for node types to be visible without programmatic intervention. (For example, when client code of the JCR API works with a node in some way based on mixin types, but shouldn't be responsible for assigning them.)
For example, using the filesystem connector, mixin node types could be assigned in the properties sidecar, perhaps with a special JSON property or properties.