I like the idea of collecting guix profiles in one place, and I also like the suggestion to not put it right in $HOME. However, I don't think profiles belong in $XDG_CONFIG_HOME, for the simple reason that they are not configuration. If you really want to follow the xdg basedir spec, they would go in $XDG_DATA_HOME (~/.local/share). Personally, I already keep my profiles in ~/.local/guix, which fits my brain well but is not xdg compliant either.
-- Alex Griffin