Canon should have released the RF mount on the M body. Granted it'd make the M bodies a little taller, but they could then reorganize the internals a bit to be a little shallower or narrower. The bigger mount would force a greater height but no greater volume for the body.
The EF-M lenses would instead be RF mount in this case. They could have the same narrow barrel size with a slight flare at the base to fit the bigger mount. Some clown on this forum I'm now blocking claims such a flare would kill sales for the lens line but I can't see how. Lenses have always sold fine getting big towards the front so I don't see why the reverse would be the case. Just like the camera, the lenses wouldn't increase in volume, and I don't think people care about the diameter of lenses in their backpack or lens shelf at home, but rather the volume.
So among those lenses, some could be small image circle as the EF-M lenses in fact are. But they could also have introduced quite a number of modest primes--which my lens line theory would call consumer or street lenses--with full-sensor image circles, for almost the same cost as small-sensor-image circle. (For instance 24/2.8, 35/2, 50/2 or 85/2.8), the cost of the optics themselves is tiny; more money is surely going into the lens barrel, IS sensors and motors, and so on.) Since these lenses would be not only for the small-sensor cameras but also preparing the way for a potential full-frame sensor camera, they could afford to introduce a few more of them. Let's say. As to why the small-sensor shooter would want them? They'd only be fractionally heavier or more expensive to buy. However they'd probably have far less vignetting. And this would hold out the promise (to the extent the consumers were even aware they were buying full-frame lenses) that the lenses would continue to have serious value used, and that their system would continue on for decades, in both ways protecting their investment.
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OK, at this point, we have an alternate-reality full of RF-mount lenses (some with small image circle) and RF-mount cameras with small sensors. If Canon never introduces the full-frame mirrorless, fair enough, no real harm done. Just keep making these small-sensor RF bodies, at some point forget the big-image-circle lenses, and just support this M-like RF ecosystem for decades.
But in fact, Canon, they DID introduce the R with its full-frame sensor. And given the above strategy, then from the very first day there's be not only several more full-sensor lenses available (lenses the RF lineup is sorely missing today, mind you!) but a bunch of compact small-image-circle that, at a pinch, could be used on full-frame bodies.
And note a special ability such a lens would have: it casts a circular image on the full-frame sensor, but you needn't crop it to a specific rectangle at exposure time. You can save the whole image, and in post processing, take a normal 3:2 cut, or a vertical 2:3. Or a square, or whatever. If the camera wasn't leveled you can rotate the image a few degrees without losing pixels. This would give a benefit to using a full-frame R body with a small-circle lens that you wouldn't have with a small-sensor body. In short this combination would be mid-way between the current R and M lines in power and resolution.
I wish Canon had gone down this road. I wish I could just put the 22/2 or 18-55 zoom on my R body now and then, when I wanted something smaller in my backpack than my 24-120 but didn't want to go buy an entire second M outfit to have more portability and more importantly invest the huge time commiting the new system to muscle memory, establishing the workflow, and so on.
