I've moved to FX, which apparently is exactly what Nikon wanted me to do, so everybody is happy now
I'm not sure now, whether I would buy a "conventional" D400. I'd have to see the specs but I'm mostly happy with the D800E/D4 for wildlife/bird situations. And in some practical IQ considerations the D800 has as much reach with quality at high ISO as does the D7100 sensor. I.e. the 24mp DX sensor is more pixel dense but the 36mp FX sensor has better low light. As always a trade-off, and a close-enough one that I may just remain satisfied with my FX cameras in the wildlife/birding department.
To answer other posters above I expect a D400 would absolutely have the second motor to set the Aperture so there would not be an issue changing Aperture in LiveView. I guess the big question is, Nikon has not moved very fast with Nikkor electronic aperture setting other than on the 800mm and PC-E lenses. When will this tide change?
I'd pretty much snap up a mirrorless D400 though, because it would offer some things the FX mirrored cameras do not. Things like crazy fps, on-sensor PDAF, etc.
I still think the small buffered D7100 presages that a D400 of some ilk is coming. As others have stated here, WHEN could be the problem. But it sounds like Nikon has a 24mp 12fps capable EXPEED 4 chip now in the works. What I found with my D7000 is that I would miss shots due to the small buffer, and the missed shots would have been the better ones. I.e. In wildlife I had to commit my D7000 buffer too early thinking I had the peak action in the can, when something even more exciting happened I was forced to watch it through the viewfinder while my buffer emptied. I'm guessing a D400 would not have this problem.