Originally posted by normhead 420 ƒ/4 with a 1.4. 600 ƒ5.6 doesn't work well with APS-c, maybe for FF at least with my 300 2.8.
A single super telephoto design will not do job. You need two lenses and/r converters... there are converter and there are converters. Good ones, bad ones, ones with factors of 1.4; 1.7; 2... many options.
The Leica lens module design was quite interesting - better results than converters and more physical lenses in the porfolio from less articles. Probably not perfect for AF/digital era.
A 2.8/300 would sell in numbers, but even with TCs it will not become a perfect 600 mm lens. Anything (much) shorter than 560mm will feel short after the oven pipe was released and requires replacement. Anything longer will be too long for APS-C if we are looking at only one lens. A diffractive optics line with 300/500 mm or 400/600 mm FL would be perfect - (Nikon did very well here, w.r.t. to price, performance, size...).
What we will definitely not see will be reincarnations of the 250-600 or a 4/600 - these lenses do not fit to the current lineup and will not sell in significant numbers.