The big one for me is shooting BIF. In the UK I can very rarely reach my optimum settings of ISO400 (or better) 1/2500, F8. I need to squeeze a bit more light from somewhere. Faster lenses you will say but that means bigger and heavier and my 100-400 is as big as I'm prepared to go. Impossible because of the laws of physics I hear you say, but is it, or have we just not found the way around the rules of physics yet.
Why F8
because in the limited light conditions I often find myself in my 400mm shots are invariably at f6.3 and I lose some pics to DOF issues. I'd happily go as far as F11 if I could.
I don't think I've ever had a depth of field issue shooting bird in flight. I always shoot the 300mm wide open so 300mm f/4, 420mm f5.6, and 600mm f/8 with teleconverters. It would have to be a very large bird at very close range to start getting issues with the wrong part of the bird in focus and I can't usually track them when they're that close! I suspect depth of field issues might actually be focus accuracy issues.
I actually made a very useful change to my control setup when I realised the other day that I never change the aperture in these situations. I previously had the rear dial set for shutter speed and the front for aperture. Then I switched the lever (E-M1 II) to position 2 to get Iso on the front dial. I am in full manual. I changed this to shutter and Iso for lever position 1 which means I can crank both to change the shutter speed and Iso for different situations whilst keeping the "exposure" the same. (Inverted commas for pedants). Not having to switch the lever makes it much easier to operate with gloves on. I have shutter speed and aperture on lever position 2 for the odd occasion where I do need to change aperture.
One other point on sensors. The colour filter array on sensors means, I assume, that about 2/3 of the light falling on the sensor is thrown away. One day, some one will come up with a sensor that can record the number of photons and their wavelength which would record all the light falling on the sensels. That would give a stop and a half improvement in performance. Maybe!