The EF lenses would vignette like crazy, but maybe less than expected. The T/S lenses are basically MF with large image circle. A sensor size where the most common or best EF image circles just touched the top and bottom ... 12mm away .. might be usable. Or less ambitious, touched the left and right ... 18mm away.
Except that the T/S lenses would then cease to be a T/S lens because the image circle isn't big enough.
For Canon to start its own MF line requires a whole new camera system to be designed. This could go down as:
* body with integrated sensor - to just supply a body with sensor and use existing lenses (similar approach to 645Z)
* body with integrated sensor + lenses - similar approach to the 645Z but only works with Canon lenses
* MFDB - similar approach to Leaf and Phase One and leave body/lens manufacturing to others
* MFDB + body - produce a body that takes existing MF lenses from other manufacturers
* MFDB + body + lenses - introduce a whole new ecosystem (return on investment possible?)
To look at it from a different perspective, who would buy into Canon MFDB?
* sports photographers - they would need to replace all of their current lenses with newer, bigger lenses and if they don't work from monopods/tripods, they would then need to. For newspapers, etc, this ecosystem upgrade would be costly without any gain as current model FF cameras deliver what's required. i.e. no buyers here.
* event photographers - aren't going to want to carry around bigger and bulkier cameras and lenses to gigs, concerts, etc.
* wedding photographers - some parts of the wedding (non-walk around shoots) are suitable for MF shooting but not a whole lot. Those at the top end of this market are probably already using MF but it isn't a big market. With the barrier to entry being so low, it is hard to see wedding photographers being able to justify the spend.
* studio photographers - this group of the market is where most of the MF activity lives today. It's not an incredibly big market (if unit sales are anything to go by) and Canon would need to be very disruptive and aggressive to break into it.
* hobbyists - just don't have the money required (or not enough of them that do!) to make it worthwhile. When the type of photography is examined further, the group that benefits is landscape shooters but again equipment will become larger and less fun to take with you. If you're doing animals birds (in flight especially) then you'll need to buy and wield bigger cameras and lenses which is not all that attractive.
* cat photography - obviously the docile nature of cats and their fur makes them the perfect subject for MF digital photography however cat photographers do not seem to be particularly fussy about which camera they use so there could be difficulty in convincing this group to open their wallets.