First, let me state that I think that the 5D MkIII is going to be an awesome 'all-rounder' camera, however there are still quite a few people who want more megapixels. The D800 has caused quite a sensation (I've even heard a dedicated medium format film fan talking about buying one and going digital) and whilst I'm sure that Canon will be happy with 5D MkIII sales, I'm think that there will also be a bitter feeling of "that show used to belong to us!" at Canon HQ.
There was quite a few rumours of a 39MP sensored prototype some months back and now I'm wondering whether Canon will be tempted to put it into a production body. Another question would be whether this would be a 1Ds X, a 5Ds MkIII, or call it something new like a "3D". I don't think that a 1Ds X is likely, following Canon's marketing line that the 1D and 1Ds have been amalgamated. Futhermore, the previous Canon and Nikon generation showed that there wasn't as much demand for a full size high resolution body as for a compact/lower price model. That leaves either a 5Ds MkIII (essentially just a 5D MkII with a high resolution sensor) or something new -call it a 3D. The former would be low frame rate (I guess ~ 3.5fps with a single Digic 5), the latter might be dual-digic 5, with 5+fps. This would allow Canon to more easily exploit a higher price point than either the 5D MkIII or D800. I also believe that there's noting wrong in marketing terms with using the "3D" moniker; it's doubtful that anyone buying a camera at this price point would get confused with 3D capabilities. It might also be a good marketing line "resolution that gives your pictures the 3D look" etc.
Yes, I know that Canon have only just announced the 5D MkIII, but this is a rumours site and so I feel that speculation is acceptable, even at this early stage! An EOS 3D would be a welcome addition that would be good marketing and satisfy demand for a "big print" camera from landscape photographers who can't justify medium format prices.