An L-series 150-600 would be much appreciated. I envy both Sony E and (soon) Nikon Z shooters for that option (in both cases: 200-600). There is more than one occasion, when I don't wanna bring my big white prime, e. g. when I go for a long hike or gonna shoot in really crowded places. Right now, there is nothing in the Canon lineup that fills the gap. The 100-500 is outstandingly sharp, but does not offer enough reach for me and has this ridiculous tc issue. And the 600/800 f/11 are great little lenses but aren't weather-sealed.
A sharp and reasonably lightweight (between 3 and 3.5 kg) 200-500 f/4 with an inbuilt TC would be a dream lens for many wildlife shooters. In fact, it is for me. Very versatile even at close range or for very large subjects, such as puffins or bison, and - with a flick of a lever - enough reach for smaller birds or birds in flight. And no faffing with attaching or detaching converters in wind, dust, rain or snow - when nature photography gets most interesting! Only the price is gonna be hefty. I assume around 15.000 USD - should we ever see this lens getting produced in the first place.