A couple of things I'd love to know : will lenses be fly by wire ? 99% of my work being on a tripod with manually focused lenses, focus by wire is for me the ultimate nightmare and is one of the main reasons I am not so excited with MILCs. I was waiting to know if Canon would go the same route as the other (though I don't know if Leica has FBW). The USM of the R-L lenses seems to tell that it will be both, USM Full Time Manual on L lenses and FBW STM on "consumer" lenses. I guess it's less of a problem with mirrorless cameras since you need power to see anything at all anyway. But my experience with my 40 mm STM has been so frustrating with manual focus, I use it only for AF handheld shots, despite it being somehow better optically than my old 45 TS-E.
Since we talk about lenses I am a bit puzzled by that recent Canon obsession wit super short macro lenses (28mm EF-M, 35mm EF-S and now 35mm R), especially considering the sub-optimal manual focus on these lenses. Plus slightly longer than normal lenses are in general more useful for close-ups.
IMHO IBIS or not is big question mark as well, from what I saw, the latest gen IBIS from Sony (will see with Nikon) work as well as classic lens IS. I like the idea of IBIS, since it doesn't force every lens to be bulkier, more fragile and the complication of the optical formula doesn't in general go with better optics. Plus as Olympus an Panasonic have shown, you can combine both. So I really hope Canon did not skip the IBIS. For me it's a real plus since I have quite a few vintage manual lenses as well. No doubt the future Panasonic FF will have it as well.
As much as I'd like to see a dual card in it, my opinion is that Canon (and Nikon) don't see these cameras as fully professional tools yet. My guess is they assume for a high end amateur market one card slot is enough, especially if the card are reliable (that is the case with SD and likely even more with XQD). Plus to be fair dual card is optimal only if the are the same speed. I personally never had a card failure (I know it can happen), but on my 5DSr, the SD is slowing down the writing so much, it's almost a joke (once again I do mostly slow photography on a tripod).
Finally about the sensor, I hope they'll have improved the 5D4 sensor, and finally removed the low pass filter.