If they bump it to 40 MP and decide the speed of a stacked sensor is not needed, then I can see them using BSI. The 1-4-type sensor in the PowerShot V1 (made by Canon, AFAIK) is BSI, and has the same pixel size as the current (but older) R7. Benefits of BSI are most evident with pixels <2 µm, but there does appear to be some modest benefit with the 3.2 µm pixels of the V1 sensor, so there should be more with the ~3 µm pixels in a 40 MP APS-C sensor.
"The number “40” would be good for the marketing department."
The good news is — I guess — is that if we extrapolate the photosite density of the APS-C sensor… Canon might eventually catch up with the 60MP FF sensors that Sony has had for a few years… as Sony is rumored to go beyond that.
Extrapolating a 40Mpx sensor to FF gives 102Mpx. The current 32.5Mpx extrapolates to 83Mpx which is already higher than the Sony 60Mpx sensor. In fact, the ancient EOS 90D and the M equivalent had 32.5Mpx APS-C years ago.
40Mpx is only 11% more linear resolution than 32.5Mpx, and each 40Mpx pixel is only 10% narrower so it's not going to make much difference in the DLA, the demands on lens resolution etc. It will make no difference to the S/N and high iso when you look at the whole image, because they are independent of pixel size. And if you pixel peep and look at the engineering S/N, each pixel will have only 10% less S/N, ie, less than 1/6th stop.
