There is some speculation that the bigger prism is due to a new square sensor. Some people ask which DA-lenses can support the larger image circle, others dismiss the idea because most APS-C lenses wouldn't support a 24*24 image.
I think the idea behind a square sensor is
not to take a image with square aspect ratio, but to switch between horizontal and portrait orientation without turning the camera. Depending on the orientation, different parts of the square sensor are used to gather the image data. As this would only require 16*24 in either orientation all APS-C lenses could be used without a problem.
If the additional costs of the larger sensor were lower then the additional costs of a vertical grip, it would be a smart and compact innovation.
Just to heat up the talk that the presumed ISO button shows in fact two squares and a totally new function...