Well, there's room left in the naming for a D400 and D500 (between the D300 and D600) ... will they be used by cheaper FX camera's or "flagship" DX camera's? Tough call.
I'm under the impression that, with the arrival of the D600, Nikon is lowering the FX cost to a prosumer level. The D500 (and D400) could be reserved for cheaper FX camera's. Nikon might reason that DX lenses work in crop mode on FX, so no need to throw away your glass; and that most of us DX shooters have an FX fantasy.
On the other hand, Nikon could mimic the FX model line-up (D4 vs. D800) and introduce the DX versions: D400 vs. D7100