Canon improving their 24x36mm models (more pixels? more dynamic range?) could consolidate their take-over of much of the former medium format sector, but will not help them to compete in the higher volume professional level markets like sports and journalism against the best that the new digital formats can offer, as shown by the D2X.
They also need something of full professional quality matching the high resolution (lp/mm) and thus the shorter usable telephoto lens needs of the smaller D2X pixels, and at high frame rates, which the 1Ds models have never offered.
The only way I can see to do this is to also roughly match the smaller formats used by Nikon, including the 12x18mm (2x) of its high speed crop mode. Copying the high speed crop mode idea with a full 24x36mm sensor is probaly too much of a size gap; it would need about 30MP to match the D2X resolution.
So I would bet on an "EF-S" format EOS-1D "sports/PJ model".
P. S. I find it ironic that Nikon's new high speed, sports oriented photography mode uses a cropped frame format of 12x18mm, just slightly smaller than Four Thirds format. After all, it been "explained" so many times in online forums that smaller formats are inherently inferior for high speed applications like sports!
I also note that the D2X sensor ISO speed disadvantage relative to the 1DsMkII seems distinctly less than one stop, so in many cases is more than outweighed by a roughly one-stop advantage in f-stop with shorter telephoto focal lengths. Of course it is Canon's move next to respond to Sony's new CMOS sensor by reducing noise even more.