A higher resolution Canon "5D Mk III", of say 30-40MP would not steal many sales from the 1DX, because the dominant market for the latter is driven by its high speed performance (frame rate, AF, ISO) performance. The strategy of avoiding cannibalization by not offering a popular new product option is the fallacy that bought Kodak down ["no digital, it will steal from our film sales"], and hurt Xerox ["the computers with a mouse and GUI, email and laser printer developed at PARC will steal from our photocopier revenues"]. If you do not "eat your own", the competition will. Nikon is doing a wise thing, offering the D800 and D4 with clear advantages over each other, serving different use cases.
That leaves the question of whether Nikon or Canon will offer a third variant: their lower resolution 36x24mm sensor in a lower spec., lower speed body, akin to the D700. I suspect and hope not: people spending about $3000 on a camera body should learn that a higher pixel count image converts nicely to an image of lower resolution, reduced noise, and improved DR. You cannot however convert in the other direction, adding resolution that is not there.