I think the api that allows for more flexibility of implementation underneath is a hierarchical multi tag system with one root of the tag reserved for the exist

I think the api that allows for more flexibility of implementation underneath is a hierarchical multi tag system with one root of the tag reserved for the existing filesystem, eg. "fs". Paths on /fs/dir/dir/file could serve as definitive storage reference and a way into the existing filesystem apis, while other tag systems can be imposed by the user in parallel trees of tags - all of which refer to the underlying fs tree under their implementations.