There are a number of different ways a camera can alternate between shooting its full / natural resolution (let's assume 70MP for this hypothetical higher end Canon FF mirrorless / EOS RF mount camera body). I am hoping that Canon will introduce new hardware with this camera, that overcomes the data transfer bottle neck
I have a reasonably sound appreciation of the general aspects of data through-put, computational power required for AF and tracking (in this case, on sensor/DPAF) and the interaction with frame rates on each of these. I do not understand all the technical details (I expect few of us outside a Canon lab/factory really do, though some folks on this forum are more clued up than others).
If this camera is a megapixel monster, of 70MP, I would be very happy with 3 fps at full resolution (or even 2 fps, as I very rarely need more than that for high megapixel settings). If Canon can provide one (or even more) high quality (i.e. decent DR and clean / low-noise images) at reduced resolution modes, that will allow it faster fps, that would (obviously) meet the wishes of many a photographer who captures images across multiple genres (as I do).
Having mRAW and sRAW are important considerations here, as the flexibility of post processing with RAW files gives more latitude than JPEGs. I see the APS-C (crop mode) as being distinctly different to reduced resolution modes. For this reason, I am keen to see both used in all Canon FF mirrorless... and I aim to keep at least a few of my EF-S / APS-C compatible lenses (e.g. the Canon EF-S 10-18mm STM, Sigma 8-16mm HSM, and potentially the very handy Canon EF-S 15-85mm IS USM etc).
I would love to see Canon pull off a great product like this... and even though it will likely be relatively expensive, this one product would serve many photographers so well. Video features / functionality / capacity is much less important to me, than still photos, though I do appreciate for others, video is very important. For me, solid build quality, ergonomics, still photo quality, longevity (i.e. shutter rated to >=200,000 actuations would be great) and photographic features are things I want.
I am looking forward to keeping an eye on what Canon will pull out of the hat! With their seriously good RF lenses, I believe Canon has a serious quality FF Mirrorless up their sleeve....
PJ