The "Pro" FF Mirrorless is unlikely to be all that much better in terms of image quality and performance because Canon doesn't seem to have any new sensor or processor technology. The last couple of FF cameras that they released use old sensors that have been "re-designed". 6DII uses 6D sensor, EOS R uses 5DIV sensor. Pro body will probably use a version of 6DII sensor if their goal is high FPS and no crop 4K. Or, it'll use a high-res sensor from existing (but "re-designed") 5DR for high MP. If they had any new sensor technology like BSI sensors, they surely would have debuted this tech on the all new revolutionary EOS R.
What will probably change for the better though is the controls (thumbstick for focus points) and dual card slots. 1080P 120FPS will probably be included no matter which way they go because that's just a function of processor speed. If a single Digic 8 can't handle it, they can just use two of them.
I'm not sure I'll ever
need better image quality than what's on modern sensors, not that I wouldn't take improvements, of course; the flip side of this is that I'm not sure I'll ever buy another camera solely because of more megapixels or better DR, though I'll certainly take it in conjunction with other useful technology.
On the other hand, I will likely wait for, and likely pay a lot more for, the More Pro R just to get a joystick =X
I would also not mind FPS being in the 6-8 range. But I'll wait to actually play with an R to see if I buy it today or wait a few months to buy the next one. The reason would not be to have a better sensor, though.
