Overall, bringing IBIS to the M lineup in a higher end body and a mid-level body would make a lot of sense to me. Also, as noted, the higher end body could have a built in EVF. You can call it a M5II, but it could also be a M1.
Canon is showing some signs of taking the M lineup seriously. Anyone that owns the M6 II knows it is actually a very capable little camera. Rumors are that they are adding lenses. A couple could be higher end.
I dont realy understand why the M series is still necessary with the R. The RP is allready super small and could get cheaper in the near future. Why not replace any M camera with a RP?
Or they could build a RF camera with an APS-C Sensor... Shouldnt it be super easy to build small APS-C lenses for the R mount? So this could easily replace the M. Whats the necessary of this series?
Sometimes it is easiest to define yourself against the competition. For the M, think micro 4/3s, many of the smaller APS-C set ups.
Plus, because of the M mount size vs the size of the R, the M lineup will always be able to be smaller than the R. Then, include the fact that Canon has always done a good job of providing a product at each price point, the M allows them to dip into those lower price points, both with lenses and bodies.