With the Imminent arrival of the Nikon D4S, I started thinking about what I really need as an upgrade to my D4:
1. More megapixels - I know this is a controversial statement, but I wish I had more megapixels on my D4. Yes, I do own and shoot the D800 and no, it doesn't suit every purpose. Why do I need more MP, isn't the 16MP of the D4 plenty? Yes, 16MP IS plenty, so let me be more precise - I wish I was able to get 16MP in a slightly denser arrangement to effectively multiply my focal length. I shoot a lot of small birds and tend to use the 1.4, 1.7 and 2X teleconverters a lot. If a D4S allowed me to shoot 1.2 or 1.3X crop mode and still gave me 16 MP, I'd be able to reduce my usage of the 1.7 and 2X teleconverters which really slow down AF and degrade image quality to an extent.
2. Move the AF-On button - They moved the AF-On button a little to the left on the D4 (compared to the D3) and it's now a little bit of a stretch for my thumb. Not unusable but slightly uncomfortable - though since I only use the this button for AF (my shutter button is set to operate ONLY the shutter) this is quite a big deal for me.
3. Better video - I'd like D4S video to have the same sharpness as D800 video. I don't know why Nikon couldn't create comparable video modes on two cameras that were released a month or two apart.
4. External control for the auto ISO feature - the thing I'd really like to have is an external control for the minimum shutter speed of the Auto ISO feature. The current option is buried inside a few menus and takes some fiddling to set, and I have missed shots trying to set this. I really find I need this even when shooting with a fixed lens - when I switch from perched birds to birds in flight for example, I need a very different minimum shutter speed and am willing to sacrifice a little ISO bump to get to it.
5. Better AF with smaller aperture lenses - my 600mm f/4 is essentially an f/8 lens with the 2X TC attached. The AF becomes very sluggish in all but the best light, so significant improvement to that would be huge for me.
6. External level control for the mic - when shooting video, I almost always shoot with an external mic that feeds into the camera. The level control for the mic input is also buried inside a few menus and is tough to get to in a hurry.
Will Nikon make all these changes? Probably not. But one can always hope!