The best thing Canon could do in this arena (something no other manufacturer has done) is to make the body fully compatible with their existing lenses - period.
That is exactly what Pentax has done with the K-01. It is a DSLR without the mirror.
Question is, what is preferable: a mirrorless that is
a) bulky like a DSLR in order to continue using existing - big! - lenses
b) compact body plus new, compact lenses plus full compatibility with existing existing lenses via adapter
Clear and overwhelming preference in the market is type b)
I would love to see one or all of the following mirrorless systems from Canon:
1.) 5D 3 in a smaller body aka "the Leica M9 killer
The very best imaginable FF-sensored mirrorless. Size between Minolta CLE and Leica M9, excellent FF sensor with fast Hybrid-AF (with FPPD-AF as in EOS 650D), incredibly good state of the art EVF, fast, responsive, fully weather-sealed, fully pro grade body, radio wireless controller built in instead of popo-up flash. Think of a
5D 3 in a much smaller form factor without mirror box and prism. Price of course similar to 5D 3.
Lens mount is really easy: Canon EM
... an electrified Leica M-mount, backwards compatible with all the great and ultra-compact manual focus M-mount lenses. Plus a small selection of new, digitally optimized Canon M lenses: 2 ultra compact highest optical grade zooms .. 24-60mmf/4, 50-135mm/4 plus 24/2.0, 35/1.8, 40/2.8 pancake, 50/1.4, 85/1.8 as ultra-compact fixed focals, all ultra-extra nano-coated and all sorts of ED, UHDand aspherical glass in them for stunning optical quality. Plus a "EF-to-M" adapter with full Canon lens protocol and AF, IS functionality ... basically a small EF+EM-extension tube - but call it "mirrorless pancake adapter", price it at 199,- and it will sell like hotcakes. :-)
2. 7D performance in a very small package - aka "the NEX-9 killer
Similarly well featured, but ASP-C sensor. Body size like Sony NEX. Same mount and lenses as the Leica killer. Same pancake adapter. On this one you can not only use EF- but of course also EF-S lenses with full functionality.
3. - aka "the killer mFT
yes that's right. Forget about another proprietary Canon mount. Just build a compact mFT body with clearly the best sensor, image quality and AF, speed, performance and EVF on the market plus a few killer Canon mFT-lenses, that quality-minded Oly and Pana users will also happily purchase.
Unfortunately I expect Canon to take the follwing alternative:
4. lame sub-mFT mirrorless ... aka "the Canon crappy-consumer mirrorless
Sub-mFT sensor-size (G1X) consumer-level body with features and capabilities at best at EOS xxxxD level at twice the price. Along with some crappy cheap and dark consumer kit lenses.