f-Stoppers ran a nice piece on the camera a couple of days ago – https://fstoppers.com/gear/nikon-d850-be-baby-nikon-d5-more-specs-rumored-190005
They indicate that the D850 will be selling for about $4,000 or about $1,000 more than the D810. Apparently, Nikon is considering doing the same thing Canon did with the 5D Mark III, it will continue to produce and sell the D810. It will use a 46MP sensor. No seen anything about whether this is a Sony or Nikon sensor, but I would expect a Nikon-made sensor.
It is more difficult maintaining high ISO and dynamic range as you increase pixel density (resolution). So far, Nikon and Sony have done some nice tricks to reduce noise at the base level of the sensor that provides a near-iso-less product. An additional issue with the higher resolution sensors is that they clearly demonstrate the limitations of lower resolution lenses.
All-in-all, I expect that it will be a fine camera though the reviewers will undoubtedly show their dissatisfaction where it doesn’t meet their expectations. Nikon has chose to not go use the Mark I etc. process of naming. That’s probably a good thing as far as controlling expectations.