According to latest rumors, along with upcoming Nikon Z6S and Nikon Z7S, Nikon may also announce a new higher-end/flagship full frame mirrorless camera in late 2020 to early 2021. The name of this new camera could be named Nikon Z8 or Z9.
Nikon Z8/Z9 is rumored to feature with a new 60MP full frame CMOS sensor, this sensor could be same as Sony a7R IV‘s sensor. Z8/Z9 may shoot full-resolution images at 14fps (mechanical shutter) with AE/AF.
The upcoming Nikon D850 successor could be named Nikon D880, it may have same features and specs as Nikon Z8/Z9. Nikon D880 will be coming in 2021, after the Nikon Z8/Z9.