When we look at Sony's 26MP 1.5x APS sensors, their new 61MP FF sensor, and their 100MP miniMF sensor we see that they all use what is probably the exact same photo site density, or pixel pitch. In other words, they are likely the same sensor, but scaled up for the larger formats.
It is interesting to extrapolate the same way with Canon's 1.6X APS-C sensors. It appears that a 32.5MP 1.6X APS-C sensor is coming from Canon in the M line. Taking the photo site density of that sensor, which is slightly smaller than the Sony APS-C sensor, and increasing the dimensions to produce a full-frame 24x36mm sensor gets us to a bit more than 80MP. In other words, a 75MP full frame sensor would actually be a slightly conservative move based on technology that Canon already has.
For real fun — and I have heard no rumors that this will happen — if Canon were to expand the area of the 32.5MP 1.6x APS-C sensor to the 33x44mm miniMF format size... it would produce an approximately 140+MP miniMF sensor!