Mere speculation, because Canon talks to me even less than to the Northrups.I wonder why the RF 100-400 when there is the 100-500. I read somewhere the RF 100-500 stays at f/5.6 until 400mm. I am not sure if that is true but if it is true why another RF 100-400. If the reports of RF 100-400 is going to be true that is another area Canon missed out IMO. They could have made a 100-400 and then a 200-600.
I suspect that the 100-400 will be a 7.1 lens with lower build quality to compete against the Sigma and Tamron offerings. It will likely have a longer minimum focusing distance as well.
As for the 100-500L hitting 5.6 at 400mm, well...sort of. If you set your aperture to increase in 1/2-stop increments, then it will be 5.6 at 400mm. If you leave it at the default 1/3 increments, the expect to see the lens go to 6.3 at 363 (result reported by Gordon Laing at CameraLabs).
The 100-X00L lenses are all about getting reach in a compact rugged package. Before the advent of the consumer superzooms, 400mm was as long as you could go without buying an exotic. Today, the 100-400 is more intermediate than long, which I suspect is why Canon stretched it out to 500mm without making the lens appreciably bigger. It still doesn't compete (in terms of reach) with the longest consumer super lenses, but it gets you a lot closer.
Frankly, I don't think Canon can do a superzoom in a compact package. There is the DO technology, but I don't think anyone has done a zoom employing it. So, when Canon produces a consumer lens, it will probably go to 600 (or 700mm), but it will be (at least) 1-2lbs heavier than the 100-500.
You pay for reach, but you pay even more to put reach into a small, compact package.