The D400 will have significant gains over the D7k, we just can see it now. . .
1. AF Module will certainly be enhanced and trump the D7k. Look at the D200->D300, HUGE leap. . .The D4 and D400 will invariably have the same module, ovbiously DX/FX differences, but will more than likely have larger VF coverage. D7k has only 39 points.
2. AF Speed associated with new module.
3. FPS and shutter life - I suspect the D400 will be on par with the D4, offering 300k shutter life and close to 9fps ungripped.
4. Dual CF (far faster/better than SDHC, I don't know?!)
5. Full weather seal.
6. Exotic body materials - those rumors of CF wouldn't surprise me; instead of plastic, use high-tensile-CF, rubber, and mag alloy?
So I think the needs of a pro will be demand a D400. . .I certainly to miss the DX POV