With the rumors saying that they are, and competition from Sony, Pan/Oly, and now Nikon, etc., Canon probably will make a mirrorless camera offering soon (just a guess)?? I just have one simple suggestion. Make it a full frame, 35mm, camera. That might specifically be what gives it an edge over Nikon 1 and Pan/Oly micro4/3rds systems, even if it means for a slightly bigger body and larger lenses. Although I must say, the 40mm Pancake at f/2.8 is rather unimpressive.
In many situations I would quite like to trade a large, heavy, Full frame/ASPC DSLR with a hulking L lens for a smaller, lighter, and easily bag-able option. I would probably take a hell of a lot more pictures if I had a more compact/discreet setup. The best option for me currently would likely be Pan/Oly, but I won't trade what I have now for a cropped system. Micro 4/3rds system is also great, and if I could get something like it in full-frame I would most definitely switch, but I don't want to lose the DOF mainly, or even the quality of sensor that I feel Canon does better for stills.
So basically Mirrorless for a small reduction of full-frame (5Dish) body size and weight, along with fast pancake prime lenses. Another idea that might be insane but is worth at least testing, two sensors and two lens mounts on the same camera so that you could walk around with two different focal length pancake prime lenses on the same system or shoot in 3D. I can do without that bit altogether, just a thought.