From what I've reading there are some other things Nikon could use beside the sensor itself, as in better processors/4K video.
Right now I'm on the fence regarding buying anything significant photography wise. I hope Fuji get's their act together so I can put up with the weight differential vs m43, if not I will possibly get the new E-M2? and few WR lenses for travel/ inhospitable areas (Fuji needs better autofocus, better video, work more with Adobe to deal better with x-trans, more lightweight WR lenses, maybe IBIS, etc).
I will want a backup for the D750 for times when my trips are mainly photographic and I'm willing to carry more; so far I had the A7ii/A7rii with adapter in mind but I will wait to see if Nikon can come up with something in the next half a year or so, be it based on NX1 technology or not.