The only ergonomic difference between the D810 and D500 is the location of the ISO button. I remapped mine to the red button so it appears Nikon agree with me and that is a better place to locate it.
Bit off topic but my guess for the D810 replacement will be the 42mp sensor (used in A7Rii), plus focus system and auto calibration from the D5/D500. I am willing to be they will ditch the flash too in favour of GPS and Wifi, which would be good. These would make me want to upgrade for sure.