Canon EF 500mm f5.6L IS.
For bird and wildlife photographers who can't afford the f4.
Like to regularly hand hold their equipment in the wild
A sharp prime weighing less than a 300 f2.8
Enough reach without convertors and their associated drop in AF speed and IQ
Just a thought
There are lots of photographers who would like it to happen. The reality is that it would likely sell for $3500, and few would pay that much. It would be a very large lens, about the size of the Sigma 150-600 Sport, which is smaller only because its f/6.3. I also doubt that it would weigh less than the 300mm f/2.8 due to the additional metal in the longer length, and the thicker metal needed to withstand more bending stress.
One of the issues I had with my 400mm f/5.6 was its length and its long MFD. A 500mm would be longer and larger in diameter. A zoom has a advantage in that they are shorter when zoomed wide, and can have close mfd's without getting excessively long while zoomed wide.
If Canon could put one out for that price, I'm sure there would be many buyers, I'd prefer a 200-500mm zoom that would be shorter when stored. I could live with f/6.3 as well, but Canon seems to have a limit at f/5.6, and won't send false exif info to a camera just to get around the limitation of the cameras.