I have a couple of Panny GX8s and a Nikon D800. The GX8 viewfinder is probably one of the best available, but the D800 optical is better (for me, anyway.) For me, that means that a mirrorless Nikon has got to have *something* that is distinctly different than what they already have, or what's the point? I would think that at a minimum, the camera would have to be much more compact and lighter and still weather sealed. The next step would be smaller lenses, with a new modern mount and what we think of as "pro" specs. I think those lenses may be hinted at in Thom's speculation.
What I'd really like to see is a modernization of the whole Nikon system, but I doubt that we'll ever see it. Nikon doesn't think that way -- they have a whole culture of upgrading, rather than of revolution. The FF aspect ratio, for example, is an artifact of antique film cameras -- they should get rid of it. Go to a 4:3 aspect ratio with a sensor bigger than the m4/3 series, some kind of automatically-engaged software that would create quick panoramas for landscape photographers who want a wider aspect ratio (with maybe a tripod head to go with it) all in a camera without a fake pentaprism housing, and about the size of the Leica 10. I don't see that happening.
As an aside, I've looked at various examples of Bernard's photos over quite a few years now. That somebody should suggest that Bernard doesn't know what tack-sharp means, is laughable.