if you want similar ergonomics you can't just replace the prism with batteries.
Have you seen the Lensrentals teardown of the 5Ds? behind and around the sensor there's very little, if any spare space. Remove the mirror, AF sensor and prism and there isn't anything else left that can be thrown out if you want the sort of computing power, connectivity and ergonomics that the 5D/D810/etc offers
The only way you'd fit more battery into a 5D sized mirrorless package would be to install an integrated slim line battery similar to that you see in tablets or mobile phones in between the pcb's and rear screen (while moving the sensor forward to the mount
The E-mount is a lot shallower than the EF mount, since there's no mirror box. Replace the EF mount with an E-mount in the same body and you can move the sensor forward a bit and have a few extra centimetres between the back of the sensor and the front of the LCD, which can be packed with electronics without changing the thickness of the camera. More electronics can go in the 'hump' which would otherwise have housed the pentaprism, as well as to the left of the lens mount/sensor. Essentially, you've moved much of the electronics from beside the sensor to behind the sensor (maintaining the same thickness), leaving the entire right side of the camera, including the huge grip area, able to be filled with battery.