Short answer, I'm betting nothing will come on EF but maybe RF. Long answer to follow:
Canon doesn't seem to want to stray from f/5.6 at the most narrow on EF as it would impact focusing success on some lower end bodies. In fact, I think the long offerings from Sigma and Tamron actually trick the autofocus system into believing that their most narrow aperture is 5.6 and not the actual 6.3 that it is, but that can impact focus success in lower light. Since the focus system on most Canon cameras has a lot of f/5.6 autofocus points, but few or no f/8 autofocus points, I suspect that any f/6.3 Canon-manufactured lenses are out of the question for EF. A long zoom with f/5.6 at its most narrow gets really big really fast, and there's a lot of competition out there from third party manufacturers so it may not be worth it for Canon to bother trying to compete. If Canon released a direct competitor to the Sigma/Tamron, it would be huge to get the f/5.6 they'd want and it would likely need to be a fair bit more expensive. I'd point to the Nikon 200-500 - it hits the f/5.6 threshold, but it's still 100mm shy of the offerings from Sigma/Tamron and weighs half a pound more. I think Canon has bet that there aren't enough people who would choose their expensive offering over the Sigma/Tamron reasonably priced offerings, especially if they had to settle for 500mm instead of 600mm.
With that said, the EOS R can focus at f/11, so those f/6.3s shouldn't be as limiting on RF as on EF. On the M system Canon has moved away from that f/5.6 threshold (i.e. the EF-M 15-45 f/3.5-6.3), so it stands to reason that they could do the same on RF. Further, I'm not sure the third party manufacturers have been able to reverse engineer the RF mount protocol, so I don't think they can make an RF mount super zoom with autofocus yet (other than having users just use the adapter), so Canon may arguably have less competition on super zooms for RF which may make building such a lens more attractive to them. It's possible, but if they do it I'll bet that the quality wouldn't meet the 100-400, and it will be expensive as hell because there's no competition.
Probably not the answer people want to hear, but that's my thinking for the little that it's worth!
