Is the new Canon a big improvement against rivals? ...... Even against the
old Nik0n D300s
http://www.dxomark.com/Cameras/Canon/EOS-7D-Mark-II
If it were all only about the sensor, then no. Every potential owner of a crop sensor DSLR would have a Nikon 24mp sensor camera.
But the camera is more than the sensor. For me, the 7DII is the first camera to tempt me to upgrade from my D300S in 5 years. More fps, more pixels, better high ISO performance and potentially better AF. The sensor may not be as good as Nikon's, but then they don't have a camera that has all the other 7DII features.
The 7D2 is an interesting move by Canon, but how is it going to affect the sale of fx canon cameras? Before the 7D2 people were moving towards the 5d3 or 6d, like what nikon have done with the d750 and d810. The 7d2 seems like a mini cut down version of the 1dx, nikon made this mistake with the d300/d700/d3 and haven't got past it since. I think canon are hoping to get nikon users to move across using the 7d2 as bait whereas nikon are looking at moving everyone to fx at present.
The flaw in Nikon's strategy is the expectation that all the people who want a DX high performance camera, for whatever reasons, will move to an FX camera because it is a similar price.
Some will, some have, but if you really want/need the type of camera the D300S and 7DII are, then anything else would be making compromises in various areas, depending on the choices you choose to compromise on.
As for whether Nikon will bring out a D400 or whatever, I doubt it sadly.
(though I hope I'm wrong) I don't think Nikon have it in them to compete with the 7DII. If they did, there would be 'rumours' starting to appear to try and deter those Nikon users waiting way too long for a D400, or anyone else wanting a camera like that, from changing, especially over the Christmas period. They may have been playing chicken with Canon over this section of the market, but Nikon may have been bluffing.
Nikon could have had that sector of the market sown up by bringing out a 'D400' with slightly updated features with a 24mp sensor a couple of years ago, but they have chosen to seemingly ignore it.
Next March could be more interesting but really once you are canon or nikon is there any point moving across and does it really matter?
Canon and Nikon may believe that users stay locked into their brand once they're in, but those days are long gone imho. If you don't give users what they want, some may upgrade to whatever compromised option you put in front them, but once you start compromising, then a whole lot of other options open up, not just DSLRs from Canon or whoever, but also a whole load of CSC cameras. For me the 7DII is very tempting.
As for something by March, it may be too late by then for a lot of those waiting/wanting for a 'D400', depending on some in depth 7DII reviews as well as user's experiences. Unless rumours of it being able to blow the 7DII away in almost every area, which you would assume anything to do with the sensor would do anyway, then people may already have gone.