Following the familiar Canon pattern of releasing big white super telephoto lenses, CanonRumors indicates it heard from one of its own sources that a 300mm f/2.8 and 500mm f/4 set of lenses will release in 2022, well after the initial release of the 400mm f/2.8 and the 600mm f/4.
Were Canon to do so, this would be the normal course of events. An interesting note, though, was that the 500mm f/4 was said to be perhaps smaller and lighter than would be expected. This may have to do with the fact that, while the most recent 400 f/2.8 and 600 f/4 lenses experienced major weight reductions in their most recent iterations, the 500mm f/4 II hasn’t yet been updated at all. Were it to get the same treatment, one would expect the lens to become quite handleable.
Camnostic was able to test the 500mm f/4 II and the 600 f/4 III lenses side-by-side, and the newer 600mm lens was just about as handleable as the 500, but for when in constricted spaces. The Canon Rumors article suggests that the lens may also be shorter, which would imply a diffractive optics (DO) element, or a much different lens design than has been used previously.