The more I think about this the more I believe that an M7 is the more likely possibility. An APS-C R body would create tremendous market confusion and splitting APS-C bodies into three different lines (DSLR, M and R) seems unlikely. If Canon truly intends to phase out DSLRs (I don't believe they do, but I'm willing to accept the possibility) then I can't imagine them mucking up the R and M lines with a mixture of sensor sizes.
It makes much more sense and far less confusing to consumers to have a single full frame line and single crop sensor line.
Canon could create a very compelling M7 to sit at the top of the line using the 90D sensor and carrying over many of the R5 features. There is nothing that the R body provides that could not be built into an M7. In fact, like the 7D, it could be essentially the same size as the R5.
They've designed the R series to offer the option of a 1.6 crop, so why put a 1.6 crop sensor in R bodies? If they proceed with a high megapixel R body, there is even less of a reason to offer a crop sensor camera. Put a 90 mp full frame sensor in and you'd got the equivalent of the 90D sensor in a full frame body, while offering all the advantages of full frame.
When Canon created the R mount they made a huge deal out of how they could now design lenses specifically for the R mount, offering things they couldn't offer in EF. Why mess with that by throwing in RF crop sensor lenses? Plus, most RF lenses would be ridiculous overkill for a crop sensor and you'd still have the confusing 1.6 factor for focal lengths.
Adding an additional body at the top end of the M line would help sell EF-M lenses and the cost of new lenses would be spread out over more bodies, including a body that appeals to the highly lucrative enthusiast market.
All EF lenses work fine with the existing adapter, just as they do with the RF adapter. So, consumers who want to use both formats would still be able to do so using EF lenses.
The only question would be whether or not they create a handful of long zooms or primes that are custom tuned to the M mount.
Canon's new RF lenses show they can create low-cost telephotos, so they could certainly build a handful of telephotos specifically for the M line. In fact, I would bet Canon engineers would be energized by the prospect of designing lenses designed specifically for the M mount and the APS-C image circle.
For Canon, the prospect of a mirrorless future with a clear cut division between APS-C and Full Frame has got to be compelling.