Admittedly, my interest in the 1Dx is purely academic – I don't expect to ever be able to afford one and I don't really need one.
But, it is interesting to see what Canon is considering for its flagship, as I think it can offer some hints down the line. The 5DIII is where I am at and where I expect to be for quite some time.
Honestly, for me, I would like to see Canon reverse the relative positioning of the sensor between the 6D and 5D – Give the 5DIV an even more excellent low-light sensor at about 22 mp and then give the 6D a 24-28 mp sensor. If you want high resolution, then buy the 5Ds. Realistically, I doubt that would happen because they won't want to give the 5DIV a sensor that performs better than the 1Dx.
As a second choice, letting the 1Dx and 5D share the same sensor would make sense.
Canon has never hesitated to let its APS-C cameras share the same sensors (7D and virtually everything else; 70D and 7DII; etc.) So, alternatively, letting the 5DIV and 1DxII share the same sensor would be consistent. There are plenty of other ways to differentiate the two.
If they bump the 5DIV sensor up above 24mp and sacrifice low-light performance, they won't be selling me another 5D. I simply have to have the high ISO performance. Fortunately, I suspect there are a lot of others in my position (photojournalists, wedding shooters, event shooters, etc.) so I don't seem them changing the winning formula.
A 7DII-like autofocus, an extra frame or two in frame rate, dual-pixel autofocus and call it a day. Nice add-ons would be touch screen; usable wi-fi; GPS, 4K.