It would be great to see an upgrade path for 5D Mark III users.
It's already here, and it's called the C100. For the time being.
If you care enough about video, and mk3's shortcomings get in the way (lack of resolved detail, no dedicated video features) the cost of a C100 ought to be a non issue.
I own both mk3 and C100 so I know their respective strengths. The C100 produces absolutely beautiful images. But having two cameras are not optimal for me. Like you Andrew, I wish the mk3 resolved more detail—at least as much as the 1Dc in S35 mode.
I don't buy the idea of a cheaper C-body camera. There's no room below the C100 for that. There is however plenty of room below the 1Dc.
A 5Dc or 3Dc that was priced between the mk3 and the 1Dx (probably closer to the latter) would be a very cool surprise come NAB. But it better deliver. I feel that Canon is a bit stuck with their more expensive models as it is difficult for Canon to outperform their top gear with cheaper models. But there is a slot free for a great C-line DSLR that doesn't have 4k, but delivers great 1080p along with stills functionality we're used to.
If they really want to innovate, they'd allow for a little nose after the EF-mount and integrate ND-filters. That's not going to happen, but the new xDc might.