Differentiation is the hardest thing for me to get my head around.
Each new model must differentiate itself from the previous model. Hence the 5DIII has to be better than the 5DII.
At the same time, each new model must differentiate itself from other new models. Hence the 5DIII must be inferior to the 1DX.
Complicating factor: the DSLR customer base has matured and is not likely to expand rapidly. That means, manufacturers have to lean more heavily on current owners to upgrade, rather than simply hope they can expand the base by attracting new buyers.
Finally, there is the convergence and advancement of technology. The clearly definable differences between the top of the line and the bottom on the line keep getting compressed. You need to make sure your $500 DSLR takes pretty great pictures, but somehow, it has to be about $6,000 less great than your top of the line model. And, you've got to slot a whole range of models in between those two, when the reality is that you simply don't have enough differences in features to go around.
So what is Canon to do?
They only have about nine things to play with:
Bell and Whistles (GPS/Infrared Trigger/Radio Trigger etc.)
For the 5DIII vs. the 1DX they can't play with sensor size, leaving only the other eight.
Leave the resolution a little above the 1Dx, but not too much (21-24 mp);
Leave the ISO Speed a little below the 1Dx (might need to be two stops less in order to add sufficient differentiation);
Some improvement in dynamic range/image quality but this can be relatively subjective and, since the current model is already really good, improvements here aren't going to be a big return on investment;
Autofocus. This is the weakest feature of the 5DII. I think it's a no-brainer that this will get better. Maybe it will get the 7D autofocus;
Frame rate. Not likely to go up much. They need to keep this low to avoid cannibalizing 1Dx sales;
Weathersealing. Same as frame rate. Some improvement possible, but probably only minimal. May not see any change at all;
Bells and Whistles. Some things could be added here that aren't that important to 1Dx owners. At the least, it is likely to be compatible with the 1Dx add ons.
Video. This is the $64,000 question in my mind. I think Canon was caught completely off-guard by the incredible popularity of the 5DII for video. 1Dx buyers are not video buyers and never will be, so Canon has the most flexibility here. I would not be surprised to see a number of improvements designed to make the 5DIII more appealing for low-budget video. This is the only segment of the market that has the chance for significant growth, and they are dominating that market. They won't want to give it up.
That's my two cents worth.