I think that a new Canon EF 400mm f/5.6L IS USM will be quite welcome. I am presently in the market for a 70-300 and am waiting for the first tests of the new EF 70-300 IS II USM to arrive. If both IQ and AF-speed and accuracy are good, I will get that. Otherwise I will possibly (have to) buy another brand.
But if it will be the EF 70-300 IS II USM, I know I will also need something a bit more powerfull. I occasionally shoot birds and airplanes, and then 400 mm on an APS-C is very good.
I have tried an EF 500mm f/4L IS once for a few minutes on an airshow. And although AF was lightning fast, I presonally found it was a bit big and heavy to manoeuvre in the crowd of an airshow. The guy who I could try it from said he would also have prefered a 400 mm, looking back at it.
About 8 months ago I thought that I had the ideal solution. I saw the
Canon EF 400mm f/5.6L USM and the reviews about the IQ were fine. I already decided that I wanted that lens, and even wanted to buy it before buying my new 70-300. The price is high for such an old lens, but it seemed worth it.
But then I saw my mistake: that lens has
no IS!
I was disappointed and horrified at the same time.
So I think a new 400mm with IS would be welcome.
Keep it f/5.6 to limit weight and dimensions.
Off course, AF must be lightning fast for the intended use of such a lens.
I want to add that I think I would like it to have ring-USM as opposed to nano-USM so that for birding I would have REAL FTM instead of the STM- or nano-USM version of FTM.