Don't forget the EF-M mount was designed so that it *could* work with full-frame sensors (it's virtually identical to the FE mount on Sony which can still work with absolutely ANY design capable of working on the EF mount for full-frame). It was only later that Canon decided NOT to use this mount for full frame.
What this means is that for APS-C, the EF-M mount is similarly capable of maximum performance over older mounts for the smaller APS-C sensor as the RFmount is for full frame.
We've seen a slight increase in size on the EF-M mount since the first EOS-M through to the EOS M5. It wouldn't take much imagination to see Canon producing a more upmarket M body somewhere in size between the M5 and a 90D. We've already seen plans for a possible EF-M mount APS-C 100-400 5.0-7.1 lens which would fit nicely with this too.
It *could* happen, so could an EOS R7. I have no idea which will come.
As you've seen with the R5 and the new 600/800 lenses, Canon are anything but predictable!
Either move, an R7 or an M7, would break a Canon paradigm; full frame [and full size] vs. compact [and crop]. People favoring one alternative will point to the paradigm that the OTHER alternative would break, and say "Canon will never break that" while ignoring the equal unlikelihood of them breaking the paradigm that's blocking their alternative.
If Canon will not break a paradigm, there will never be a full size crop, and that applies to both an R7 and an M7. The arguments one camp is using apply against them as well, which is why this discussion has been fruitless.
As Michael Clark pointed out over and over and over again yesterday (to the point where I nearly lost patience with the repetition of the same rant), Canon has never produced a lens for the M that wasn't a
single particular outside diameter; to make the hypothetical M7 useful for the current 7D crowd, they'd have to break with this (and I wish they would, for other reasons). [Or that crowd would have to buckle down and (*gasp*!!!) use a fragging adapter, but they would be "stuck" with EF lenses that way. The horror!] The logical fallacy here is that Canon is incapable of changing its mind or, perhaps, coming out with another "line" of cameras that use the M mount. That way they could keep their single diameter fetish for the "entry level" line but create another line that isn't bound by those rules, and the lenses would likely be completely interchangeable between the two lines (and why not, the line would be a marketing distinction, not an engineering one). So I could see a semi-pro EF-M mount camera coming out, though the model number might not start with a single "M", maybe PM or MP or something like that. This would let them keep their paradigm largely intact, by supplying a loophole to it. Rather than the EF-M
mount being restricted to compact cameras, it would be the M
line of cameras and lenses, which
just so happens to use the EF-M mount, being restricted to compact cameras, leaving cameras outside that line free to use the EF-M mount as well.
The R mount, on the other hand, seems explicitly intended for Full Frame, Full Stop. They could do something similar here, creating a new line for the crop sensor camera, I'll call it the RC line for convenience. Though I hope that if they do they don't come out with RF-S lenses, or if they do, they don't create/restrict them to a variant of the RF mount.
So you'd have two present, and two potential, marketing lines of lenses.
R cameras and R lenses, using the RF mount, with a full frame sensor.
RC cameras and RC lenses, using the RF mount and a crop sensor. The RC lenses, though only imaging enough to cover a crop sensor, would be interchangeable with the R lenses, with R cameras going into crop mode if an RC lens is attached; there is no special RF-S mount that MUST be used with an RC lens.
MP cameras and MP lenses, using the EF-M mount and (of course) a crop sensor. These could have all the weather sealing, ergos, and build quality of the current 7D, though I can imagine a lesser model roughly corresponding to 90D, that doesn't, being sold alongside the MP7. There'd be no restriction on the sizes of these, and big telephotos and/or superzooms would abound here. Lenses would be fully interchangeable with the M line of cameras.
M cameras and M lenses, using the EF-M mount as well; this is the current bunch of cameras whose name starts with M. The lenses could (and often would) be used on the MP cameras, but nothing that cannot be done in a 61 mm diameter would be sold as an M lens. (Hmm, how close can they get to F/1 on a 61mm focal length? Obviously they can't get all the way there because the lens must have a wall of finite thickness.)
Thus you could have FOUR distinct lines while still having only two mounts, and it's reasonably "tidy." There's no necessity for building redundant lenses just to put a different name on them. Now I expect Canon would chose RC or MP but not both; but nothing precludes them going back a few years later and saying "yeah we should do the MP" (or "yeah we should do the RC") if they see a potential market for it.
Love that MP 100-400 lens but all you have is an M200? No problem, put it on, and ignore anyone who snickers at how the camera looks because they can't snicker at your pictures.
Need an 800mm lens for your R, and all you can afford is an RC800mm? Well, if your R has high resolution, get the RC 800mm and get cropped photos, or for the framing an R 800mm would give you, buy the RC 500mm.