In addition to a D7000, I have a D3s and D800e. For about a year, I was strictly and FX user, having sold a D300s and obtained a D700 to accompany the D3s.
What I found was that I missed some features of DX, particularly as a smaller, lighter travel camera, and the "reach" of a DX sensor. So I swapped the D700 for a D7000, as it was the best DX choice at the time (late 2011), although I would have been willing to go with a D300s again.
So, I could just shoot the D800e in DX crop mode with DX lenses, but it is still a bigger and heavier camera for traveling. And it doesn't get you the burst speed of the D300s or even D7000.
The D7000 does not have the "pro" control layout of the D3s or D800e, so it is always a consciousness shift to use it.
So a D400 (D300s with ISO, MP and EV enhancements) would really fit the bill.
I don't think the D300 series was meant to get people on an upgrade path to the D600/800, as they did not yet exist. The D300 related to the D3 as a DX, small body pro camera. The D700 came along to fit the niche for a smaller body FX camera.
Now we have a full line of FX camera, including the "introductory D600, but no up-to-date DX "pro" body. If Nikon introduced one, I would be glad to buy it and get rid of the D7000.