It seems the most logical path for Canon would be to make a 5Dmk3s/x or some such thing as an answer to the D800.
The need for a bulletproof body such as the 1D class for even most pros is dubious.
Canon is quite conservative and the demonstrated success of the D800 fairly calls out for response.
The high status 1D bodies and D4 bodies do not garner nearly the attention (and certainly not the sales) they used to. The 5D/D800/D600 bodies point the way to a segmentation of the higher volume FF landscape to come.
While I agree with you in that Canon needs a higher MP body, the 5D Mark III is actually much more successful at sales than the D800. So the masses of consumers/pros do not consider high MP necessary. When you say "demonstrated success of the D800 fairly calls out for a response", that is actually not true, because in sales the 5D3 is selling way more than the D800. I'd say the D800 therefore, wasn't all that successful compared to Canon.
For the pros, it would be nice for, myself included, to have a high MP, low fps, low ISO, 1D body like the 1Ds Mark III. Replacing the 5D Mark III already, considering it's sales success, makes no sense. It will most certainly be a higher-end body.