I have been wanting to see something like an RF 500mm f/5.6 L for a long time now. Basically a slightly longer and updated EF 400mm f/5.6 USM L. that lens is still tack sharp on an R1 or R5. It was the “budget” L lens for the “amateur” nature photographer or weekend warrior. It’s pretty much always with me when I travel. It’s only slightly longer, but skinnier than my 70-200 f/2.8. An RF prime version of either a 400 or 500 mm f/5.6 lens would definitely be able to come in cheaper than a lot of these zoom offerings, and would be pretty light too. The zooms are cool, but they need some sort of middle L series telephoto prime. I have the EF 300mm f/4 L IS USM as well, and I like it, the stabilization was useful on the older bodies from time to time, but the 400 f/5.6 is still way sharper. The 300 is also kind of loud focusing. At least the adaptors work flawlessly on these R bodies, so I don’t see myself selling any of my FL, FD or EF Canon glass, but I would definitely be interested in a lightweight, RF telephoto prime in the f/5.6 area.
Though, if this is small enough, and the price isn’t astronomical… We shall see.
I still use my EF 400mm f/5.6 USM L a lot with my R1 and 1DX MK III. Probably that and my EF 180mm f/3.5 Macro, RF 100mm Macro, and the EF 600mm F/4 L IS USM III are my main 4 lenses I cycle through (leaving so many others sadly left on my desk).