Both the Nikon Z8 and the Nikon D850 replacement are rumored to have a 60MP sensor:
There will be 2 more Nikon mirrorless cameras announced this year, one of them should be high-end/pro (Z8?)
The Z8 is rumored to have dual SD and CF express cards slots, vertical grip option, pro body, higher price tag (previous rumors suggested two CF Express cards)
The high-end mirrorless Z camera will be announced before the Nikon D850 replacement
Nikon D850 replacement is not expected in 2020, maybe in 2021