It would be nice to be able to ask rsync to output its verbosity and progress information as xml, so that a GUI could easily parse the info and show it in a GUI.
As a maintainer of one of that rsync-using-GUIs I would find that also very nice.
But I don't see an advantage with XML. To complex and not human readable. Cost a lot of resources (think about sustainability) to parse.
For me JSON seems to me as a good minimal consent.