Yes, "file" tag looks redundant, as the file info will get repeated in the File group. "item" seems okay. or maybe "token", but token is a datatype in W3C XSD. I don't know whether that can be the reason for not using it.
Or probably, root element can be given the name "metadatas" (plural) and the individual file elements as "metadata" (singular), with an attribute giving directory and filename concatenated.
exiftool a.jpg -keywords -X