I think its too soon for 5D3 refresh but this would be my list for me to buy a 5d4.
1. Blinky AF point
2. Linked AF point Spot Metering
3. 1/250th Sync speed bump. (1/500 would be even more sweet)
4. Built in RT transmitter
5. 7 FPS bump and improved buffer for dual card writing. (pitiful if you use the SD card slot currently.)
6. Dual CF or SD, just pick one or the other.
7. Add dual custom buttons like the 1Dx besides the lens mount.
8??. Did anyone notice the second curtain sync C-fn through PC sync missing in the mk3? I was positive this was on my 5Dc or the 5D2...
9. Make the Rate button do something else beside's rate and protect. :/
10. Slight screen bump in resolution/size and/or Touchscreen with optional toggle.
11. Lastly, Any ISO/IQ/Resolution bump is nice but these other features are much more important to
Numbers 1 and 2 would be great. Your number 4 item is odd, who here would want dual SD only? I'd venture to say most, if not all, users with cameras at this level are well invested in CF cards.