According to CR, first Canon super telephoto lens for RF mount is in developing, and it will be a diffractive optics (DO) lens for Canon EOS R (B&H/Amazon/Adorama/Focus Camera/WEX/ParkCamera).
The focal length for this first RF mount DO lens would be 500mm DO lens. Canon is also rumored developing RF 300mm f/2.8L lens, but not coming anytime sooner.
via: CR