We are told that Canon is currently working hard on a new RF 500mm f/4L IS USM Super Telephoto Prime lens, which will be the first super telephoto lens for the RF mount.
In addition, a new Canon RF 300mm f/2.8L IS USM lens is also in development. Canon tends to announce super telephoto lenses in pairs, so maybe we’ll see that Canon RF 500mm f/4L IS USM & RF 300mm f/2.8L IS USM lens will be announced together.
Although we don’t know the exact announcement date at the present, we expect to see the Canon RF 500mm f/4L IS & RF 300mm f/2.8L IS USM super telephoto lenses after the release of Pro Canon EOS R body, which is rumored to be released in early 2020.
In addition, the new Canon’s “holy trinity” RF mount zoom lenses will start shipping before the end of July 2019 (the development announcement was back in February):
The official announcement date of all the three Canon RF lenses will likely be before the end of July 2019.