Sony already has its A7RII sensor working but in the pixel binning mode for 4K. It fully reads the cropped s35 image for an oversampled 4K (off 15 MPX). Panasonic's DVX200 will also have an oversampled 4K from ~ 13.5 MPX. Sony's has a horrible rolling shutter effect, as reading 15 MPX simultaneously is obviously very taxing on a $3,200 camera and, to do 8K, it'll need to read 33 MPX rather than 15.Anyone curious should look up Panavision's "Dynamax" sensor... sounded like a revolution, apparently they could never get it to work.
My engineering expertise ends there but I have to believe this is eminently doable, considering that Samsung's NX1 reads its 26 MPX sensor simultaneously - for an oversampled 4K too - in a $1,300 mirrorless body. The rest is on the processing, memory, heat sink, etc.