The RF mount has seen some extreme lens designs, with Canon pushing into new territory with options like the revered 28-70mm f/2L. It seems they have no intention of slowing down, as another interesting design has emerged, this time one sure to please sports and wildlife photographers.
A recent Canon patent has emerged for an RF 100-300mm f/2.8 1.4x Extender lens. While such a lens is not unheard of, as both Nikon and Sigma make 120-300mm f/2.8 lenses, this would be the first such full frame with a built-in teleconverter, which would effectively make it a 140-420 f/4 lens when engaged.
I do wonder if this is why we have not yet seen an RF 300mm f/2.8 lens, as Canon may be exploring the idea of replacing it with a lens such as this. That being said, just because a patent appears, it does not mean the product will eventually be brought to market, but I am confident we will see some sort of 300mm f/2.8 lens for the RF mount eventually, whether a prime or a zoom.
Having owned and used the Sigma 120-300mm f/2.8 for several years, I can say it is a tremendously versatile and useful lens, and I wouldn't be surprised if Canon chose to go the zoom route to fill out the shorter supertelephoto focal lengths. I suspect such a lens would be quite popular. Hopefully, we will hear more soon!