Since day one of this announcement, I have been imagining and saying that the R3 is to the 1D(or R1) as the R6 is to the R5. Anything they do with the R3 is meant to compete with certain Sony bodies, cost more than the r5 but be better, but also leave room for the r1 to be the top-of-line flagship. This R3 sounds like a curious hybrid of R5 and 1D.
While I use that r3<r1=r6<r5 logic to predict a likely 20-24mp sensor, I also realize that the sensor is a new BSI stacked sensor. Because of that and my familiarity with BSI sensors, owning a Sony Rx1Rm2, I would predict that a BSI sensor of 46 to 56mp is possible, also giving 30fps RAW. One can also conclude that if a rumored R1 is being produced, and given a BSI sensor too, then it may well have a similar mp count and be a scorching 40 to 60fps. I guess that whatever this camera has, the speculated-on R1 will be a bit better, and vice-versa.
Be advised, this is just speculation, PURE speculation.
I am a street photographer who shoots commercial work from time to time, so this camera would fit me well, I think. The 5DSr I had was great for commercial, but terrible for street work or any shot with ISO over 200, or any shot with movement while shooting slower shutters than 1/500.