Since, the current 400 & 600 primes are EF lenses adapted to the RF mount; their due for a refresh. Before the world cup would make sense, but that doesn't mean they will do it.
As for a switchable teleconverter; I would prefer a completely separate unit. That way it can be used on other lenses and when it's needed. A built in teleconverter adds weight, length, and cost to the lens. I only need 1.4, not a fan of a 2.0 teleconverters ( my personal opinion). They have a working switchable teleconverter from the EF 200-400mm, that has shown reliability for years. Why haven't they adapted this into a separate teleconverter; there is no doubt in my mind it would sell. I would buy it.
In my opinion, since the introduction of the RF mount Canon has put the long glass needs on the back burner. During this time, Sony & Nikon have developed better options and it's time for Canon to step up. The classic example is the gap between the 100-500mm and 400/600mm primes in the "L" series lenses. If you want upgrade to 600mm or faster glass, your ONLY option is a $13 to 14k prime. Then there is the mythical 300-600mm that has been teased for years. This would be a great wildlife lens, if their smart enough to make it a variable aperture of f4 to 5.6. For those of us that shoot in low light and want better bokeh; a fixed f5.6 aperture is unacceptable at the 300 to 400mm range for the anticipated price.