In 1998, I conceived of a filesystem that essentially equated directories and files with elements in an XML document. The idea was that the filesystem API would essentially be no different from an XML object model API, filesystem paths would merge with XPath and there would be a smooth transition from directory structure to the internal structure of files (e.g. if a file /foo/bar had a section "baz", it would be addressable as /foo/bar/baz)
I called this filesystem Plan X as it seems a natural extension of Plan 9 with the obligatory "X" for XML.
When I later looked at ReiserFS, I saw a number of similarities.
Recently, I read Grant Miner's New Advances in the Filesystem Space which talks about many of the sorts of things I originally envisaged for Plan X.
I think if I did this now it would be called Plan Py and everything would be a Python object.
libferris looks like a very nice project along the lines of Plan X.