I agree that the D700 does not have a replacement in the D600 due to the reduced feature. Nikon markets the D600 as an entry level FX camera, vs the D800 advance amateur/semi-pro/pro and D4 (pro cameras).
I don't think that the D700 (and D300/300s) will have traditional replacements as Nikon have several lines now in production: D3--- series (entry DX), D--- (amateur DX), D---(advanced amateur DX), D6-- (FX entry), D8-- (advanced amateur, semi-pro/pro FX), D- (pro FX).
The D3/D700/D300 bodies seems to represent the older technology and older naming convention.
I think Nikon with have DX-only bodies as D---and amateur FX as D--- and pro as D-. Given that FX bodies can be shot in DX mode albeit, lower MP but with cleaner high ISO performance, I wonder if Nikon needs a D300/300s replacement.