I would be up for a 5DIV.
Aside from a better image, what else would others want in a 5DIII upgrade to make it worthwhile? Hmmmm.
I for one wold like better/faster focusing, and it would be great if they included an intrinsic automated afma adjustment like Focal, or better, if there was a way that it could be done in real time all the time.
I wonder if it might not be a little early for a 5DIII replacement, because a lot of people have not fully ammortised the purchase cost of their 5DIIIs yet. When the 5DIII came, most people with 5DIIs sorely needed to replace them, so there was an almost guaranteed revenue stream for Canon. I think Canon would want to delay a launch long enough to guarantee a sufficiently strong stream of upgrades.