Presumably much of this is based on an intuitive feel for the various lenses mentioned. Unfortunately intuitions often misleads us. Particularly when physics and economics are involved. For example, the concept of the "fat boy" 200mm. This seems to imply that by some design trick Nikon was able to achieve higher magnification with a shorter and larger diameter stack on that particular lens. However if one looks past the intuitive feel to the facts they reveal the opposite. Of the "exotic" telephotos the 600mm f4 already has the shortest, "fattest" dimensions. Consider the following comparison of equivalent focal length vs physical length of three of the "exotics":It would be great for the long lenses 400 or 600 F4 to be light and smaller exploiting the FL feature even more by becoming smaller fat boys, now shortening may effect the bokah we love at F4 which looks like a 300 f2.8 at 2,8 on the 600mm lens, so increasing the diameter and shortening the length brings some of it back, ie: like the designee for the 200mm F2 fat boy, so in my dreams more a fat boy 600mm F2 would be the ticket, also with all having built in TC of 1.4. and x2 that works perfectly.
As to 200-600 it becomes a 3 times magnification which is ok as apposed to the 200-500 at 2.5 to 1, this may be a trade off unless they make it FL super light and fat boy which allows them to peel it back to F4 better still F2.8 is possible if they want, I mean the 70-200 FL is an awesome example of the concept just exploit the concept..
200mm f2 actual length = 203mm
400mm f2.8 actual length = 358mm
600mm f4 actual length = 432mm
So relative to the equivalent focal length the 600 f4 is already the "shortest and fattest". Which begs the question, if it's 432mm long why is it called a 600mm lens? And why do other optics like binos and spotting scopes use a magnification factor like 6x, 9x, etc, rather than focal length? I won't bore those who don't care with the explanation. Those with inquiring minds can Google it for themselves.