At this stage of camera development, considering what is already available in the market, built-in WiFi would be a must. I would love to see a WiFi remote control APP similar to what is available for the Fuji X-T1, where you can completely control the camera from a remote device such as a smart phone or tablet.
What remains to be seen is what improvements, if any, will be made to the video capabilities. The camera will probably have a tilting view screen, but is a coin toss as to whether or not it will come with an anti-aliasing filter.
With specs like these, the D750 is beginning to look like the perfect, all-around pro camera. The Df was crippled by having the 39-point AF of the D610 instead of 51-point AF of the D4s/D810. Also, the Df is a specialist camera built to be more about nostalgia than practical use. The D610 with its 24MP sensor is a fine camera, but without the advanced features of the top pro models. The D750 appears to be the re-incarnation and update of the popular D700, able to define its own slot in the Nikon pro camera line-up. Price? More than likely in the $2500 range to position it between the D610 ($1850) and D810 ($3300).
All of this is speculation, of course, but it gives us something to dream about while waiting for the next photo assignment.