Let the format be optionally extended with an origin name, to be able to define packaging format in a more fine-grained way. An example is:
[ 'flatpak:flathub', 'rpm', 'flatpak' ]which prefers apps from the Flathub Flatpak remote, if it does not exist, then chooses an RPM version of the app, and if even that does not exist, then chooses a Flatpak version from any remote.