39 MP worries me, but not for the reasons expressed by some in this thread. It's because of what it says about Canon's target market for this camera, which is not the market segment that bought the 7D and 7D2.
How do you think that the market segmentation is different between 7D/7Dii and rumoured R7ii?
The 7D/7Dii appealed above all to outdoors/wildlife photographers, and by the standards of the day they had pretty much everything. The R7 doesn't - as a mirrorless replacement for the 90D it was quite good, and incredibly affordable, but as a 7Dii successor it wasn't even close.
The single biggest fault with the R7 for fast-moving wildlife is the slow sensor, and a higher MP count makes a stacked BSI sensor less likely. Not impossible, but less likely. The problem manifests itself mainly in ES mode, but you have to use ES for high fps bursts and quiet operation - essential for some wildlife. (Have you ever compared the sound of the R7's mechanical shutter to the 7D's?) The smaller, less robust body was a big step back for people working outdoors, in the cold and sometimes with wet/muddy hands, or gloves.
I'm not forgetting that your question was about the "rumoured R7ii", not the current R7. If Canon does manage to fix all these things in the Mark II, I will be absolutely delighted, but moving up from '90D' spec to '7D' spec, with its attendant shift to a new price bracket, would be a big move and while I remain hopeful, I'm honestly not optimistic. History tells us that it seems much more likely that the R7ii will bear a close resemblance to the R7. And if so, it will once again fail to appeal to the '7D market'.
Just as a point of reference, the OM-1 has all of these things, with a slightly smaller 20MP Micro Four-Thirds sensor. The launch price of the OM-1 Mark II was USD2,200, compared with USD1,500 for the R7, so while the camera that many of us want is proven to be achievable, it would come at a high price.