In a recently interview with Nikon executive officer Kimito Uemura by NHK World Japan (Video above), Nikon executive confirmed they will bring new Nikon mirrorless cameras before Spring of 2019.
Previous rumors and report also said both Nikon and Canon will release their full frame mirrorless cameras before CP+ Show 2019. Rumors also said the prototypes of Nikon full frame mirrorless cameras already in testing, so we can expected a development announcement of Nikon full frame mirrorless cameras around Photokina 2018 in September.