*Update*: We don't believe this lens is coming in 2020 anymore.
We have been told by a great source that Canon plans to announce an RF 70-400mm f/4.5-5.6L IS USM lens sometime this year. Though we don't exactly know when.
Back in September, we saw a patent for such an optical formula. and it makes a lot of sense that such a lens would hit the RF mount sooner than later.
The same source also said to expect it to be about the same size as the recently released Canon RF 70-200mm f/2.8L IS USM.
The image for this post is the EF 100-400mm f/4.5-5.6L IS II USM