Please let me post a very wild idea... I've thought about it before and it looks plausible to me.
Pentax at the moment is very experienced with implementing the SR technique. Now let's for a second imagine that the new body will have a new SR system that does not only work in the X and Y axis, but also in the Z axis.
Think about it. This means that the sensor can be used for focusing. If it is not dependent on the lens, then focusing while filming can be virtually noiseless (the focusing noise is a problem for the video mode). Possibly the focusing could also be faster.
So this is my idea I got after having seen the large prism housing, maybe it needs to be that large to incorporate a focusing mechanism too.
As I said: just a wild idea