Until equipment certification and flight test standards are developed and implemented I don’t see how a rating system could be possible. The entire pilot rating system is predicated on pilots demonstrating their skill and knowledge against well documented, published performance standards.
IMO, any test standard they might come up with needs to have testing performed without benefit of GPS. As things are now, with 30 minutes of “practice” the average 5 year old could fly as good or better as most when using GPS assisted flight. That was sort supported when a UAV outfit developed a new operating system and invited a Cub Scout troop to come in and fly the simulators. Most of them flew almost as well as many of the company’s experienced pilots.