It will be really interesting to see what Nikon does in terms of new lens designs vs. support for existing lenses. To me the whole point of mirrorless is to reduce both size and weight of the camera/lens combination. The longer Nikon teles and some of the fixed aperture zooms are quite heavy as well as large and this would potentially negate the use of a mirrorless system.
For me, size and weight are much less important. A little smaller than DSLRs is nice, but too small is not great either.
Excellent performance and ergonomics are more important to me. On-sensor focussing weems to have the potential of providing excellent auto-focus, but great lenses have to be big enough to accomodate the optical elements and motors. For professional cameras, that's OK with me.
I expect that Nikon's first mirrorless will be a tour-de-force mirrorless D850-class camera: high res, high dynamic range, lots of computational tricks like focus stacking and eye-focus. But what will the second and third be? I have a couple of thoughts (but absolutely no what Nikon will do.)
One possibility would be a less-expensive, smaller, lighter camera targeted at travel. Think full-frame Leica CL. Not really designed for ultimate performance, not really matched to long lenses, but very well-balanced with wides and the shorter zooms people most often travel with. Perfect for street photography, but only adequate for Formula One.
Another possibility, if Nikon wanted to make a splash in the video market, would be a low-light mirrorless that bests the Sony A7S. I don't expect this, but if they wanted to move aggressively into that space, a camera like that would do it. If it had a PL adapter and played nicely with non-Nikon wide lenses, that would be even better.
A third would be an expensive, medium-res, high-performance camera in the D5/A9 class with excellent weather sealing, class-leading data rates and autofocus and the like.
In the mediate-term, a lot depends on how clearly they set their design goals for the first several models. One camera for all doesn't seem like a strategy for success.