It appears to me that there never will be a D400 DX However, I think there may be a new smaller FX camera in the future. With the introduction of the the D700, Nikon started using that body for FX and has continued with the D800.
I think you should look for the D7100 to be the next "top of the line" DX camera. Nikon might use the 24MP Sensor but probably not hold back any features. The body may be slightly larger to accommodate more controls, but I never expect to see another DX Sensor in a D1-4 body.