A source at Nikongear spilled out the possible Nikon D850 specs. And it really seems like it will NOT use the same Sony A7rII sensor. Instead it will feature a Sony made (and Nikon tweaked?) 46 megapixel Full Frame sensor. It is highly likely that the new Sony A7rIII which is probably going to be announced in early 2018 will feature an even higher resolution sensor (something between 60-70MP) and not this new 46MP sensor.
These are the other specs from the same source:
10 fps (without battery pack)
together with fast AF this will be a sports capable camera with 46 Megapixels resolution
(appears to adapt something of the D500 concept for FX)
NOT compatible with MB-D12, there will be a new dedicated battery-pack
They expect the first samples to be available in October
Pricing will be about 1000 Euros above the D810 which will remain available in parallel
Thanks Bill on Dpreview forum for this.