Someone needs to come out with a current high quality 400mm f5.6 prime. I would switch systems for a high quality handheld bird lens.I think that there are a lot of people interested in a new version of the 400F5.6.... I would get it....
I love my 400mm f/5.6L... Is there anything this "lot" of people need from it that it doesn't have/do?! Weather sealing, sure, but asides from that I love it's relatively light weight and small size without IS/zoom/wider aperture - coupled with the 5D3 sensor's ISO handling this thing's even incredible when losing daylight!
I guess closer focusing would be nice but that'd maybe ruin the magical optical formula!
It's a good lens but it could be significantly sharper, and it desperately needs IS. Try a 300mm f/2.8 +1.4xTC and you will see what a tack sharp modern lens can do. Even my Sigma Tele Macro f/5.6 400mm is sharper than my former 400mm f/5.6 L. That is not just a personal view, you can check the MTFs on Photozone.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M5 15-45, f/2 22, 11-22, Samyang 8mm f/2.8 fisheye