According to report from Japanese website Sankeibiz, Nikon currently is developing their full frame mirrorless cameras in rapid pace, most likely released in this Nikon’s fiscal year (End on March, 2019. And also said Nikon will announce at a major overseas trade show – Photokina 2018 in September.
Below is what Sankeibiz said: (Translated)
Nikon also strong against single lens reflexes also advances the development of mirrorless at a rapid pace and clarifies the policy to release in the current fiscal year. In some cases it will be lazy as “I will show off at a large-scale overseas exhibition” (industry official).