In an interview with Keiji Oishi, Nikon's UX Planning Department Manager, a few details were shared on dpreview that typically don't get talked about by a company before the actual product announcements:
- A flagship Z model will appear "within the year".
- The goal is surpassing the D6.
- It will debut a high-resolution, stacked sensor.
This corresponds with the information I've heard: possibly late summer/fall official announcement, probably 45mp with a new Nikon-designed sensor, probably 8K video, definitely targeting the highest end user. (Since there are plenty of rumors of new cameras floating around at the moment, for those wondering what the two just-registered products that are likely to be announced next are, I believe them to be a DX mirrorless camera and the D850 DSLR replacement.)
The full interview is quite long, and has a fair amount of interesting information in it, so I highly suggest you read it on the dpreview site. Again, it seems strange that Nikon is making marketing noise post-CP+, not at CP+.