The day is coming when metadata will be implemented as an XML document in accordance with an industry standard (or individual custom) data type document.
It would seem straight-forward to include an XML document in place of vorbis comments in a slightly-modified version of the flac file format. I don't know about other formats and how adaptable they might be, as most are hampered by large corporate self-interests.
Comments are welcomed...in fact, desired. I know of at least one standards organization that is considering this approach.