Hmmm, I wonder if the following is possible? ..... In theory, if Canon wanted to preserve backwards compatibility with EF lenses for a future "main" mirrorless system, I suppose they could make a FF mirrorless camera mount, give it the form factor of an SLR (like the mirrorless Samsung NX1 or Sony a3000) and include a modular adapter/box that has a mirror, a pentaprism, phase detection sensor, and an OVF. Without the "box", the camera is a FF mirrorless which can mount its own unique lenses; with the "box", it accepts EF lenses and turns the mirrorless into an SLR.
This has already been done before: The Sony A7x mirrorless series has an adapter, LA-E4, that has an SLT mirror inside to allow the mounting of Sony SLR lenses, with continuous phase detection autofocus. Add a modular grip in addition to the adapter, and voila, mirrorless becomes SLR-like