For me:-
1. The sensor, AF, and flash systems of the new D5 - much like the D700 had the equivalent smarts from the D3 - now justifiable given the large % price increase of the D5 over the D4S
2. The build and ruggedness of the D700, and its frame rate or better; not fussed if the weight goes up a little, so long as it is "good weight".
3. Improved focussing screens to support MF lenses, which are designed for ease of installation - like the old FM2 and FE2 were designed.
4. Continued support for pre-AI lenses, including stop down metering
5. Shift the memory card door to the hand grip - away from the bottom
6. Dual QD cards
7. Use the same battery as for the D810 - this implies a larger hand grip - which I would welcome.
8. Replace the wretched front control knob with a standard control wheel integrated into the hand grip.
9. Include a voice recorder to support in-field documentation and audio note taking
10. Articulate the rear LCD - similar to the Olympus OMD E-M1, and also make it touch sensitive.
11. Keep the rest
I quite like the look of the current Df and most of its controls but I don't believe for one moment that it has to look exactly like a FM2 or FM-3A or any other film camera. For goodness sake it is a digital SLR that should be the best blend of traditional and electronic controls with a UI for stills use - but in designing a Df Mk II Nikon doesn't have to be a total design slave to the past either.