For me it needs to have:
1. the Exmor-like low ISO DR
2. A7R II quality oversampled 4k video and not processed to be all mushy and waxy and DNR'ed, with some basic usability features like mini zoomed 100% view box to aid focusing, focusing peaking, zebras; wouldn't hurt if they made it 10bit 4k and RAW HD enabled to one-up the Sony a little (who will probably be moving forward and close to an ATR III by the time this 5D4 ever manages to arrive)
3. i'd like 42MP at 6fps (and it should, if the MP count is on the above 25MP side of things, offer a RAW crop mode, not some useless JPG crop mode like on the 5Ds that does nothing to help fps or buffer performance or card or HD storage space) but with the 5Ds release I have a feeling they will hold back to 24MP in which case I'd hope it would do 8fps so I get some more sports action out of it to make up for the lesser reach for wildlife/sports and lesser detail for landscapes
4. not make or break but I'd like spot metering to AF point for sure and RAW histo without needing ML would be nice, although Canon has trained us to feel like that is asking far too much so I barely dare write this #4
i'm not quite sure #1 will make it into this camera (i would've though to no way, but the recent interview gives it a slight hope) and they sound awfully negative on 4k and decent video on consumer DSLR so I'm thinking to just go A7R II now and not wait yet again
If it ends up say 25MP, 8fps, mushy 1080P 8bit with no usability features, the same old DR then I'd not only not get, not only not just stick with Sony additions as tie-overs, but finally give up my awesome canon lenses and UI and just get some D820 that does it all (I assume a D820 might be basically a DSLR version of the A7R II, only with a bit more fps, certainly in the RAW crop modes). If it did a few major things then I'd not go to Nikom but just add Sony for time time being until finally Canon gets it all together whether it be 5D4 or not until 5D5.