If it does happen it will shorten my wishlist to 35L, 135L and 180L Macro.
I dont like zooms as their image quality cannot approach that of primes. This holds true if the design year are within months of each other.
Zooms are heavier than primes at the furthest focal length and max aperture.
400/5.6 with IS would be a killer light long lens.
It should sell for the same price of a 70-200/2.8 IS II.
I'd be using it at 400mm all the time so a 100-400 would be useless for me.
My sigma 10-20 at 20 is incredibly sharp as is also my 24-105 and 70-200, there is no descernable difference to my 100mm 2.8, 50 1.4 and or 400 5.6. Modern high quality zooms are excellent and produce ultra sharp images and are no way 'inferior' in image quality to primes. Unless lenses are pushing the extreme boundaries of optical performance and measured in a lab then I'm sure there will a difference but in day to day shooting, nah.