Just to state the obvious, but it may help channel speculation into one of several streams:
The R1 will either be:
1) merely QUANTITATIVELY superior to the R3 and competition...
2) a GAME-CHANGER in one or more ways...
3) it will be a SAD TROMBONE, not even as good as the competition.
Since 1987, Canon has always tried to out-spec the competition. Nikon gives you a 180/2? Canon will give a 200/1.8. Nikon has a 55/1.2? Canon gives you 50/1.0, and so on. I'm not following the market that closely so what would the ante be in order to at least meet Sony and Nikon?
-- pixel-shifting: now that we have IBIS, we have a camera that can literally move its sensor. So what say we move it 1/3 of a pixel at a time and take a second exposure... again and take a third... then down a 1/3 a pixel and take a fourth... Several exposures can thus be combined to produce a higher-resolution image.
-- 102MP is available from the Fujifilm GFX 100 for $9999. The R1 may be pretty close to that price. In fact if you count the likely sales volume, $7999 would basically be that price. And 100MP has a nice "quantity has a quality all it's own" feeling to it.
-- Sony A7R4 and the Sigma fp L have 61MP so this is the minimum we could possibly have and still be in category 1.
-- 5.76MP viewfinders are used by the A7R4 and Fujifilm GFX 100. The R1 has to match that, or beat it somehow such as update rate. The A1 has 9.44M and 240Hz, however. If Canon's going to exceed ALL of the competition, that is the target.
But Canon's also tried to change the game a couple times. The change to EF mount cost sales in the short term but took Canon from 25% to 90% of the pro market. USM motors came from nowhere to make that autofocus strong, high-torque, self-locking, and silent. You can add pellicle cameras and tilt-shift lenses as well.
One idea for a game-changer mentioned above is global shutter. My expectation is that we may see the sensor Canon patented a couple years ago that has two charge accumulators per pixel and that can switch between them. The patent outlined how this can be used for global shutter (switching to charge bucket B momentarily while taking your time to read out all the buckets As), to double dynamic range (let B pick up photos every 1us out of a ms, taking in 1/1000 the light but also having smooth motion trails in most cases), and a couple other tricks. If it has this shutter, then even 50MP or less would still be a game-changer.