We know with the most certainty possible that an RF 200-500mm f/4L IS USM is in the pipeline, and we've talked about it numerous times during 2023.
We have been told that the lens has been delayed until the second half of 2024. The source claims part of it is meeting demand for the RF 100-300mm f/2.8L IS USM, as waiting 6 months for a lens isn't the best situation for company or consumer.
This fits a timeline for the lens to be showcased at both Euro 2024 in June through July and the Summer Olympics which take place in July and August of 2024.
As far as big white lenses, an RF 200-500mm f/4L IS USM would be one of, if not the most popular of the bunch. Even at the predicted price tag of “who wants to buy a kidney?”