Whatever this camera is called, it definitely won't replace the current 1D4 for a lot of people. It would be a huge step down for most wildlife and some sports photographers who use telephoto lenses. Suddenly you're going to have to be 30% closer to your subjects or have to crop and get less pixels on your subject than with the current 1D4. Although I have absolutely no interest or need for a fast full frame camera, it sounds from this thread that some people want such a contraption. I hope Canon makes them happy.
That is why I doubt a 1DIV replacement would have 18MP full frame. What sounds more plausible is the concept of the 1DIV replacement being a full frame camera with a 1.3 crop mode that yields 18MP. That would translate into just under 30.5MP in a full frame.
18MP full frame would probably result in a revolt of sports and wildlife shooters. If you use a 300mm on an APS-H sensor, you would need a 400mm on full frame. Just look at the price difference between a 300mm f/2.8 and a 400mm f/2.8.