I know there is a lot of interest in a revamp of the 16-35 f/2.8 II that has the same IQ as the 24-70 f/2.8 II.
I really don't think it is going to happen anytime remotely soon though, and here is why:
Time between 24-70 I and 24-70 II: 10 years
24-70 I designed in 2002.
24-70 II increased size of front element, and total number of elements
Time between 16-35 I and 16-35 II: 6 years
16-35 I designed in 2001.
16-35 II increased size of front element, total number of groups and elements, new coatings, etc
Difference? Only somewhat better, nowhere near the improvement 24-70 made.
16-35 II is 7 years old.
Given that the 16-35 II is a fairly recent design, being released the same year as the jaw-dropping 85L II, and the fact that ver2 was not largely better than ver1 despite larger front element, more total elements, new coatings, etc - indicates to me that 16-35mm f/2.8 is extremely difficult to get super sharpness from the wide end without a bulbous element. Every example of a similar zoom range I've heard people trumpet as being super sharp had that bulbous element.
IMO, for reportage and event photography a bulbous element is undesirable. It sticks out and is not able to handle as much rough and tough action as a regular lens; you don't even have the option of attaching a protective. People/objects banging into your camera and all. And if you use them, no screw in ND filters or CPL with a bulbous either, instead requiring a contraption for ND filters.
So while a 16-35 f/2.8 with much better performance is likely possible, it likely would also require a bulbous front element. The minimal improvement despite the size increase and number of elements increased between v1 and v2 makes it look to me that that sort of design is getting near as good as it gets.
If anyone can point to a non-bulbous 16-35 f/2.8 that destroys the 16-35 II in image quality, I would be interested in seeing it. If not, that might be your answer right there.
A lineup that would make sense to me:
17-40 f/4 - Budget
16-35 f/4 IS - Landscape photography
16-35 f/2.8 II - Event photography/reportage
14-24 f/2.8 (or 12-24) w/ bulbous element - Extreme landscape photography
If Canon came out with a 16-35 III, even if it looked as good as the Nikon 14-24 I'm sure some landscape photographers would be disappointed because it didn't go as wide... So I think that would be a bigger hit than a 16-35 III.