My craziest prediction for this camera. (Basically, what maybe
could be, but knowing Canon, definitely won't actually happen.)
The camera would have a movable sensor as some have suggested, and also a mirror like a DSLR. The viewfinder is, at its heart, an OVF, but it's a hybrid design similar to Fuji's X Pro rangefinder cameras. So it's an OVF, but there is an EVF inside the viewfinder that can turn on or off. When the camera is acting as a DSLR, the EVF is mostly off, but maybe overlaying some info on top of the view through the OVF. When the camera is acting as a mirrorless, the EVF is on all the time and the mirror is locked in the up position.
When you mount an RF lens, the mirror locks up and the sensor moves forward to the proper position for an RF lens. The camera acts as a mirrorless. When you mount an EF lens, the sensor moves back to the proper position, and you get the option of locking the mirror up and using it as a mirrorless, or using it as a DSLR with an OVF.
Won't ever happen, but it's nice to dream.