I already started receiving small bits and pieces about the new Nikon mirrorless camera and here is the recap (old and new info combined):
The official announcement of the new Nikon mirrorless camera is rumored for shortly before Photokina while shipping is expected to start a month after Photokina (there were already multiple reports for an announcement in the second half of 2018, see also this and this post)
One new camera and two new lenses will be announced first (for Photokina) with another batch of lenses following shortly after (4-6 months after Photokina)
I was told that Nikon is currently developing a large number of new mirrorless lenses and this is a priority
The most likely scenario is for a full frame mirrorless camera first (APS-C model will probably follow later, but I am still not sure about that)