No matter what the other features are, it ultimately cimes down to the sensor. That's the reason people bought the D810 instead of the D4, D5, D750 or some other camera
The D800's sensor beat anything else out at the time, and, on the D810, remained competitive against Sony's A7r2 while making up for the 5Ds's megapixel advantage with its huge DR advantage.
But will the D820 have the same advantage over the 5Ds2 and A9r? Almost certainly it will lose in resolution - 46MP vs a likely 70-80MP for Sony's new sensor, and likely at least 60MP for any 5Ds replacement. And Canon has remedied its deficiency in sensor DR (the 5D4 and 1Dx2 having DR competitive with similar sensors from other manufacturers) while Sony has made great strides in AF since the initial A7r. The D820 won't be launched into the same empty space as the D800 - it will be up against two competitive and better-resourced rivals.