I think we established above that Nikon does not intend the D600 to be the D700 replacement. It's the entry level prosumer FX body, parallel to the D7000 in DX. The D800 is the D700 replacement, like it or not. The question seems to be whether Nikon will fill the D300 gap with a new replacement of similar performance. I agree with Stan's points that a new D400 won't be a DX parallel for the D4 or the D800. But, it sounds like a lot of the shooters here would be perfectly happy with their current D300 specs + a D7000 (or slightly upgraded version) sensor. That is probably doable for about $1,700.
Will Nikon do it? Are there really that many D400 customers out there? Especially since FX is now affordable for just a few hundred dollars more? Really, if you don't shoot things far away, and don't need the crop factor, there is no reason to prefer DX other than some of the walk-around lenses are smaller and lighter. And, having shot film for years with mid-level lenses, that advantage is overrated. Maybe Nikon doesn't think there is much of a market for a dedicated amateur field sports and semi-pro wildlife camera.
Whatever they do, it will be based on their market research business model, and not on the wants and needs of us Nikonians posters.