this is a good thread I was just thinking about that yesterday. I went from the 7D mark 1 to the 5D mark III. It was a no brainer from a standpoint of IQ and resolution as well s full frame goodness.
I updated from M1 to the M3 canon mirrorless because it was resolution change, Wifi, add'l features like built in flash and tilting LCD and EVF option, as well as redesigned grip.
Likely the 5D mark iV would offer similar boost in features.
Well here goes:
1) Focus point illuminator "fix". I call it a fix since it used to remain illuminated but they removed it!
2) Wifi. This makes up for lack of a tilting LCD (which I DON'T want because I want it rugged) since you can use phone or tablet as a viewfinder. Makes sense.
3) High seed sec for shutter. 1/250th at least.
4) "erase range" for erasing image son card. My M3 has it but not the 5D!
5) focus peaking with ADJUSTABLE peak settings. I noticed the fuji's and Sony's have a higher "clip" than the canon M3's mirrorless focus peaking. the former clips in and out of Peak more quickly and allow you to "snap" into focus more visibly. This should be adjustable for the user.
6) latest and greatest, if not better, live view focusing via sensor. whatever canon calls it, the shift pixel thingie.. make it a whole different focusing option that video users can use..
7) adjustable/removable focus screens. with the new manual zeiss lens, manual focus lenses should be accommodated.
1dX level weather sealing. The 5D should just be a smaller 1dx when it comes to weather sealing. Period.
9) have the 1Dx and the 5ds/r's image based intelligent tracking sensor that uses the dedicated processor.
10) 1 cfast and 1 Cf. The Cfast is so that buffer limits ar less of a limit since it can right faster to card.
11) make it 24 MP. then release the 50 MP version with better electronic shutter to lessens cam shake as well as a sensor based stabilization to enable hand held shooting.
12) the usual sensor DR/high ISO improvements that come with each sensor revision.
13) usb 3 out for tethered shooting,
I think that covers it.