40mp, 4k video, Raw video, 1080 @60fps and a 19pt AF and they'll sell twice as many cameras as the d800. I don't know why everyone is trying to turn the 5 series into something it isn't. It's obviously for weddings, portraits, landscapes, and especially video. I expect the biggest improvement in the video specs. They OWN the dslr video market..and know that's what made the 5d2 successful.
They can sell a million without coming close to the 1Dx specs.
Which leaves no real replacement for the 1Ds3. Anyone who used the 1Ds series is after megapixels, and won't tolerate inferior AF or weather sealing either. Without a full-featured 5D3, there's no upgrade path for the 1Ds3 except Nikon.
And good luck to Canon if they think they can then make a 1Ds with essentially the same specs as the D800 and sell it for $7k.
So, for Canon, the choices are:
a) Hi-res, full-featured 5D3, competing directly with Nikon, possibly losing a few 1Dx sales (not many, since people would want the 1Dx for frame rate, just as some people bought the 1D3 instead of the 1Ds3)
b) Hi-res, limited-feature 5D3 - lose the 1Ds shooters to Nikon, likely lose many of the 5D2 crowd as well (those who don't shoot video and want good AF and weather sealing in a hi-res body)
So, either they possibly lose a few 1Dx sales (but gain a lot more lens sales by preventing migration to Nikon) or they lose a large segment of their market to the D800. After all, not many pro photographers care about video - that's what videographers are for.