I've been a lurker for about a year, finally decided to make an account. I bought my 5D Mark III last year. I really don't have any significant complaints about the camera, nor do I plan to upgrade immediately even if the 5D Mark IV has all the bells and whistles we've been hoping for. That said, I do have a list of features that would make the 5D Mark IV more compelling.
Sensor: 28-36 megapixels and 14-15 stops of dynamic range. We know Canon can crank out the megapixels now, but we'll have to wait to see whether Canon is willing to put the C300 Mark II dynamic range tech in a stills/video camera. I'm 90% certain they'll put it in the 1D-X Mark II, but less so that they'll put it in a 5D body.
Software: These days there is no reason for any camera to not have spot metering linked to AF point, use the metering sensor in AF tracking, have focus peaking, zebras, intervalometer, etc. Shove it all in there.
File Management: This is an area where the 5D3 is just dumb. We have two card slots, but you have to sacrifice your buffer to use them at the same time. The buffer drops from 33 RAWs with just a CF card to 16 RAWs with SD or SD & CF. Why can't that be done in the background? The camera should write as fast as it can to the CF card while shooting, and then while it's idle, copy photos one by one to the SD card.
Ergonomics: Replace the M-Fn button with the 7D Mark II's lever switch thing.
Radio & Wi-fi: Radio communication with RT flashes would be sweet. Having it built in may come at the expense of range compared to hotshoe mounted radio trigger, but I think most people would be okay with that. Wi-fi isn't something I personally care about, but if it's possible to use 2.4 Ghz spectrum for flashes then it's also possible to use it for Wi-fi.
If Canon throws all of those features & fixes into the 5D Mark IV, I would probably buy it within a year of release. If the sensor still has the same old 11ish stops of dynamic range, I'll just wait for the 5D Mark V.