Well, no one has any experience with 1DX yet (well, there are probably some Beta testers), so it remains to be seen...
But Canon says that new sensor technology and Digic 5+ processors makes the 18MP of the camera equivalent in quality to much higher resolution cameras.
We'll just have to wait and see if that's true.
But I'm sure that they had to limit the size of the image files to coax the ultra high frame rate out of a FF. I bet it was a considerable challenge getting a FF-size shutter and mirror mechanism to operate at those speeds, too.
Wouldn't be surprised to see another FF 1D model with a huge MP count, but slower frame rate... and then a 5D Mark III based upon similar/same sensor but with "scaled down" focus and frame rate performance.