According to recently rumors on CR, Canon currently is working on a RF 300mm f/2.8L IS USM lens for Canon full frame mirrorless cameras. This lens should coming along with RF 500mm f/4L IS USM lens sometime in 2020.
According to the source, this RF 300mm f/2.8L IS USM lens will come with a “new and unique” feature for super telephoto lenses. Yet no details available. Stay tuned.