@DriesOeyen, first, the interface name cups will stay the one for this purpose, for printing via CUPS. The interface is not ready yet, still under development, at least the design seems to be finalized now.
If you make a Snap only for systems which are all-Snap and so never have a classic CUPS, then you do not need to worry much. Simply let it plug cups. The CUPS Snap listens on both the standard domaon socket and on it own alternative socket. A Snap plugging cups will always send jobs to one of these and on the completion of the interface implementation this will not change. The parts of the interface which are not yet implemented only concern confined application Snaps downloadable from the Snap Store to be installed on systems which have a classic CUPS.