I think the resolution of the next body would depend on what they intend it to be. I know the word professional gets thrown around a lot, but if they intend it to be an all-out professional sports and photojournalism type of camera, then I would not be surprised if it was lower resolution because photojournalists have no need for massive 40mp photos, in that realm low light performance is king, and higher resolution just makes the pixels smaller.
But, I'd be surprised if they were ready to go all the way to releasing a 1D level of camera in the EOS R line yet. Those guys are largely not ready to drop their DSLRs, and I dunno if Canon is ready to make a mirrorless that performs like a 1D.
I would bet that it's higher resolution, more features, kind of a spiritual successor to the 5D4, since what we got now with the EOS R is a 5D4 sensor which honestly is a little out of date now, with more of a 6D level of features.