When you consider the size of the Leica M10 you know that it's very reasonable to think of a Nikon full frame DLSR the size and style of the FE2 or F3. There is no technical obstacle at all once you've got back to the good old clever and sleek design, without any idiotic pop-up flash, without any useless button or knob, without that pesky soap-box style molded polycarbonate outer shell.
You are assuming they want to make a body for manual focus lenses and do not want to provide a big battery. There are a few technical obstacles I would think or we would have seen it done by someone already.