What would Canon bring to the MF digital market that is not there already, and would the resultant perceived value support pricing that would make the line profitable for Canon?
Folks get excited over the technical aspects of things, but one does have to remember that Canon is a money making concern, and unless the product can be supported by the market... its a no go.
I for one cannot see an "EF in crop mode" as part of any MF camera. If you can afford the MF camera in the first place, and have the EF lenses to use in "crop mode", you almost certainly have a better performing camera body that accepts those lenses.
I can see a large number of things Canon could bring to medium format, most obviously a much lower price but also more advanced AF, faster FPS and liveview.
By far the most obvious EF lenses to use would be the TS-E's, very useful lenses for a high megapixels landscape/architecture shooters and MF current does not offer many options, espeically when it comes to lenses as wide as the 17mm TSE. The shift maybe somewhat limated but still provide a fair bit of use at a reasonable price by MF standards.
I would agree with the earlier posts that a MF mirrorless would be the most interesting option, espeically if Canon is looking for a mass market at a lower(sub £5000 anyway) price for rich amatures aswell as pros.