just out of interest, what do people expect to see in the mythical D400 that isnt catered for in one of the current crop of Nikon cameras.
Just like when the D300 came out I'd want the best DX/APS-C camera in every way.
In no particular order, and overlapping with some of what Dr_O wrote;
- 8fps at full sensor (14-bits) more fps at 12bit would be nice
- RAW buffer of at least 25-30+ images
- Supports some new faster memory card write speeds (for faster buffer clear)
- Dedicated AF-ON button
- AF system at least as good as the D800, even better from the D4S
- Shorter mirror blackout time than the D7100 (affects AF at max fps)
- Faster AF processing CPU (affects AF tracking in difficult light and AF performance when using 52pts or 3D mode)
- Larger body style (like D800)
- More of chassis built of magnesium (including lens mount) like the D800
- 10-pin connector for compatibility with other pro accessories/cameras
- Viewfinder shutter (incredibly handy when shooting from tripod)
- Next generation DX 24MP sensor (probably small improvements over the D7100)
- New video capabilities (enabled by EXPEED 4) - probably 4k video, not sure what else. I have no interest in video but 4K would be a big selling point if they implement it properly
- 1CF and 1SD slot like the D800 (better compatibility with older pro cameras)
- Shutter rated for higher duty than D7100
- Compatible battery to D800, option for compatibility with D4S battery with grip
- 91,000 pixel RGB sensor for exposure metering (D7100 has 2016 pixel sensor)
- NPS-approved as main body like D300 and D800 are (D7100 is only approved as a backup body) - only matters to pros who belong to NPS
Definately an integrated grip, its 2014 these things should be integral by now ,,all cameras now have rotating screens ,but charge for a grip to hold the camera vertically ! the bolt on jobs arent stable enough for some tripod work
I don't want a grip all the time, so I wouldn't want an integrated grip. A separate grip gives the people the choice.
All cameras certainly do not have rotating screens now.

A nice thing to have, but if it was to compromise build or weather proofing then no thanks.
The Nikon grips are apparently very well built if you were worried about movement. Most tripods can be tilted 90 degrees though if the Nikon grip is not sturdy enough.
For any new camera to replace a D300S I'd want it to at least equal what the D300S has already got, and hopefully improve in all areas where possible. A D300S replacement would have to be better than a D7*** by a long way because I doubt the replacement would be updated for many years, if ever.
Saying all that, I've virtually given up on the D300S being updated, as Nikon and Canon have been staying away from that level of camera while flooding the market with lower level cameras every year or so, and going FX crazy. That doesn't leave much room for a D9300.