CanonRumors.com proactively polled some previous sources, seeking information on Canon big whites in the pipeline. Responses indicated two were on the way, likely in the second half of 2021, and that one would be a 300mm f/2.8.
The timing is interesting, as Canon currently does not offer a wide aperture super telephoto for its RF lens lineup, forcing R5 users to adapt older glass, and this has led to some compromises. While the new R5 body beats the pants off of Canon 1D series in terms of the number of pixels captured per second, as well as tracking while using the viewfinder, some adapted lenses, like the EF 500mm f/4 Mark II cause lower framerates. The adapter itself pushes the center of gravity of the lens further away from the body, and the older lenses cannot integrate their image stabilization with that of the IBIS system in the camera.
Canon R shooters can purchase the $13,000 600mm f/4 Mark III, but that too is an EF lens, although lighter and with fewer compromises. Many will make do with the excellent-but-dim RF 100-500 zoom.
It may well be that the most anticipated releases in the latter part of 2021 will be the lenses, and not the R1 body, as forum dwellers have so far struggled to compile a sizable list potential upgrade points from the R5 to justify a 1-series body class upgrade.