A RF 70-400mm would be a lens I would love, particularly if it will be relatively compact and lightweight, with great optics. If it extends, and that means that when non-extended it is more compact and portable, that is ideal for my uses. I don't need the lens to be faster than f/5.6 for this focal length (when I need truly 'fast glass', I use primes, between f/1.4 and f/2).
Currently I own the Canon EF 70-300mm L Is, which is a great match on my 80D (and previously lived on my 7D). It has great IQ, and the size meant I take it with me on many photo outings, day trips and longer duration travel to many locations.
If Canon can come out with a RF body that meets (at least some of) my main wildlife photography needs, I will likely jump to their mirrorless FF (i.e. RF) platform. Most of my main RF photographic needs include a suitable EVF for tracking fast action and decent FPS and sufficient megapixels to allow some decent cropping of smaller subjects, e.g. birds.
Canon, it looks like our long-term relationship may well continue well into the future.....