I have multiple Chinese sources reporting about an upcoming Nikkor Z 600mm/6.3 PF lens that is expected to be about the same size/weight as the existing Nikkor AF-S 500mm f/5.6E PF ED V R lens. The aperture could also be f/5.6 (unlikely). The lens is expected to be priced around $4,000. The weight should be approx 3 pounds (1390g). The official announcement is rumored for October 10th – maybe a development announcement? The lens is currently being tested.
Please note that I do not have reliable information about this lens – those are second-hand rumors coming from China.