Of note is that because it has no grip (which usually houses the battery), the battery had to be moved so there is very little space inside the camera. That meant there was no space for IBIS. Nikon is considering an FX version based on customer feedback.
I would like a FX version styled on the Nikon FE2. I would give the camera an MD-12 style grip but not the height of the FE2 with MD-12. This would give more room inside the body proper for in body image stabilization. I would keep the camera as small as possible but I would not sacrifice a preview button and function button in the traditional positions and an AF-ON button.
Nikon might offer a battery booster or maybe not. The reason I might not is to keep the body a bit smaller and lighter. Since the camera would not be aimed directly at sports/action/wildlife photographers an extra high frame rate probably would not be that important.
I'm not sure a booster makes much difference for frame rate on a mirrorless body? I'm probably thinking in terms of a DSLR here. A booster would make a difference I'd think for video run time. The EN-EL15c would be a good choice for a battery. I don't mind carrying spare batteries for my D850 so I would not mind carrying spare batteries for the new camera.
I would very much like a backside illuminated sensor for low light performance and please no low pass filter.
Dave