What ever Nikon announces in 3 weeks Sony will counter with by announcing the A7Siii.
That makes no sense on so many levels: designing a camera takes years, and Sony won't have any possibility to “counter” Nikon for quite a while. (Not that it needs to, Sony is the 800 lbs gorilla in that particular space.)
Also, Nikon has traditionally been using Sony sensors and Nikon's image processing algorithms are also a known quantity (not that Sony knows them, but it knows full well what look they go for), so Sony knows
exactly what kind of IQ Nikon will be able to get. In the past, Sony and Nikon cameras have been using the exact same sensors, and I expect this to be true here.
Lastly, Nikon is also good at designing and making lenses, and the problem Nikon has is obvious: a rather small line-up. Of course, Nikon, just like anyone else, will offer an adapter for their F-mount, so that won't be a problem, unless you want a “native” lens.
The big question is: what is Nikon's mirrorless camera optimized for? Is it video? Is it still photography? What is the UI like, all touch screens and a few buttons or a retro UI (think Fuji)? IMHO that will decide the camera's fate. Unless Nikon pulls of a miracle, the people at Sony need not be worried too much I reckon.
If all things are equal, though, it may come down to handling. I have tried Sony mirrorless cameras and I always disliked how the UI worked and how they felt in my hand — especially once you put nice glass on them. Just for this reason, I would prefer a Fuji-style retro UI. But of course, other people have other preferences, and fortunately, there are options that fit most people's preferences
Canon is waiting until Nikon announces to see what they have. Nikon is in the worst position here. Both Sony and Canon can counter Nikon’s first attempt.
Again, the product cycles are way too long for that to make any sense. Canon has the same problem, and if it aims for a different niche than Nikon (say, Canon focusses on video capabilities), then they may not even compete in many markets. In most traditional areas all cameras are already “way more than good enough” (high ISO performance, fps continuous shooting). The only two areas where I expect a regression is AF performance (especially when compared with a Nikon D5 or a Canon EOS-1d(X)) and battery life.
Isn't already Nikon countering Sony? Sony has already proven themselves in FF mirrorless.
Not only that, Sony knows a few of the essential parameters if they supply the sensor for the new camera. Nikon will be in the unfamiliar position that it is not one of the two big guys but rather the new kid on the block trying to get a piece of the market.