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What you're describing would be a D600S (for speed). It wouldn't fit into their line, as the D700 originally sold at $3,000.
The D600 is US $2000.
The D800 is US $3000.
The D800E is US$3300.
The D4 is US $6000.
People already struggle a lot between the D600 and D800. They would have to get the price to $2,500, and slip it into the line as an upgraded D600, between the D600 and D800.
A true D700s would have the same sensor as the D3s, use the same D800 form factor, batteries, grip, etc. yet have the internals of the D3s. That would probably hit at $3500.