As for the 300-600; I just don't understand why people prefer a fixed f 5.6. If they don't want their Aperture to not change set it to 5.6. Obviously DOF considerations depend on the the size of your subject, focal length, distance to subject, and aperture. At 600mm, 5.6 is good. I don't expect them to release a zoom with same aperture as their f4 600mm prime. But at the 300 to 400mm range, I would prefer to be able to use f4.0 for low light or bokeh. For the price; f4 at the 300 to 400mm range is what I would expect.
This lens is going to be expensive. I would guess the range to be between $6500 and $10,000 ( between the Sigma 300-600 and Canon 100-300). I will be very surprised if it's less then the Sigma.
Aperture wise, a fixed f5.6 only gives you 2/3 of a stop advantage over the 100-500 on the long end ( although it is at 600mm) and NO advantage on the short end. Both Nikon & Sony's "consumer grade" 180/200-600 lenses are f5.6-6.3. Their approximately $2000. Here's my internet review prediction if it's released as a fixed f5.6: Canon 5.6 300-600mm, Is it worth double the price of the 100-500; probably NOT!
I was seriously considering the 300-600mm lens when it was f4.0-5.6, but I'm not going to pay $$$ for a fixed 5.6 lens. If they release the 400 2.8 with a built-in 1.4; I'll spend the big money and pre-order it.