I sure hope they won't make the D400 24Mp. Unless they can do that without any compromise!
With the entry level D3xxx series moving to a new 24MP sensor, two things are almost certain:
1. Future higher level Nikon models will also by 24MP or more, except ones intended specifically for very high frame rates like the D4.
2. In situations where one prefers less than 24MP, say 16MP, for the sake of less per pixel noise, downsampling from 24MP will do that just fine. (There are other options too, like applying suitable NR, another way to convert "surplus" resolution into lower noise.)
The ability to choose between less noise and more resolution by downsampling is the reason why designing multiple new sensors of higher and lower resolution in the same format makes almost no sense now, with the main exceptions being that high frame rate scenario and possible cost savings for lower level models. Especially with the latest Sony/Nikon sensors, where visible noise in the final displayed images is almost always dominated by photon shot noise rather than read noise from the camera's electronics. In the approximation of noise being solely from shot noise, downsamplng will exactly reproduce the per pixel noise levels of getting the same final pixel count from fewer photosites.
P. S. Not to say that I necessarily agree with the priorities of Sony and Nikon making the new generation of 24x16mm sensors 24MP and relegating the previous generation 16MP offerings to lower level and older models; just noting that this is what they seem to have decided to do, for better or for worse.
P. P. S. 24MP probably fits better with the new emphasis on video at 1920x1080, because the 6000x4000 pixel count allows 3:1 down-sampling and/or sub-sampling in each direction. DSLRs with lower pixel counts seem to produce 1920x1080 HD video with less than the implied resolution, because they must first use something like 3:1 down-sampling/sub-sampling to some lower resolution level and then up-sample to 1920x1080.