D600S with...

D600 with D7100 AF, 1/8000th shutter, full weather-sealing, and one-touch magnification and I'm not sure it's a replacement for the 600 or a whole different model at a higher price point.  D600 still a viable entry model at around $1900,  then this new model at around $2200.  All complete speculation and probably highly unlikely, but I'd like to see it.

The interesting thing will be the IQ comparison between these 2 models. At low ISO, it's possible or maybe even probable that the D7100 will resolve more detail.  High ISO, DR, and shadow noise are the unknowns, but I don't think it's impossible for the D7100 to be at least the equal of the 600 iin IQ, or close enough, which creates an odd situation of a lower end model being better (better AF, shutter speed, sealing) than a more expensive model.  DOF really becomes the only difference.  We'll see.