Maybe, the image circle would be big enough on some EF lenses (maybe just the TS-E lenses). I've played around with a RZ67, and it's really a pretty huge MF system. The film plane in 60mm x 70mm. The current digital MF offerings from Pentax (645D) and the new Leica S are both 30mm x 45mm give or take. Although it's bigger than FF's 24mm x 36mm, it's not THAT much bigger. Is it?
A 67 MF sensor would be astronomical to be sure, but maybe the 30x45 sensors won't be too much trouble. After all, the Pentax 645D is about the same price as the 1DX.
Historically, I believe, the benefit of MF wasn't just the megapickles. It was the increased dynamic range, lower noise, smoother gradations, aspect ratio, and that sort of thing. So, we might not see a huge increase in MPs compared to current offerings.
That being said, my hope is that Canon does come out with a MF system or a proper 1Ds replacement that has higher dynamic range (e.g., 16bit).