This real world review of the D750 shows what the spiritual successor to the 5D3 (whatever they name it) needs to target. http://petapixel.com/2014/10/14/nikon-d750-review-nikon-youve-created-monster/
PS I'm not interested in a flamewar. Just adding a data point for what the competition's refreshed body is offering.
To me, a 5DIV should be 6552x4368 (28.8MP - the short edge is the same pixel dimension as the 5D classic's long edge) dual-pixel with greatly reduced read and fixed pattern noise compared to the 5DIII. It should be 7fps (28.8*7 ~= 20.2*10 so the same pixel throughput as 7DII), and have a similar AF system as 7DII. Personally, I'd like the popup flash (the Nikon D750 and D810 have it so no whining from the "it would make the hump to big" or "it would be too fragile" crowd). Same video features as 7DII except include UHD as a 1:1 pixel (1.7x crop) option. This avoids any need to pixel bin or resample and so shouldn't be too difficult to implement (so, no whining from the Canon engineering crowd). While we're at it, add that same option to the 7DII through firmware (1.425x crop on that camera). I'd tolerate 24fps which would leave the pixel throughput the same as the stills burst rate, but 30fps would be better). It should also have the nifty 7DII features such as the strobe thing, the flexible viewfinder thing, and the focus mode lever thing.
Price it around $3k and it'll be a winner for sure.