According to a report from Nikon Rumors, it seems that the Nikon D810’s replacement has been rumored for a launch towards the later part of July and will be shipping out to customers in September. No specific dates have been mentioned, but Nikon Rumors thinks that is likely, or if there is a delay it might be pushed to August instead.
The camera in question is expected to be called the D820 and according to the rumored specifications, it will feature a 45-46MP sensor, improved low and high ISO, a newer and improved version of SnapBridge, a tiltable LCD display, two memory card slots, and it will use the same AF system as the Nikon D5. There will not be built-in GPS unfortunately.
Take it with a grain of salt for now, but the Nikon D810 was originally announced back in 2014 which means that by DSLR standards, it is due for an upgrade.