Some Random guesses:
1. Entry level, Z5ii or Zf. I guess the later has higher possibility. According to Thom Z5 does not sell as well as Nikon expected, while the production cost is quite high considering dual SD card slot / IBIS / sealing. So a Zf / Z6 market segregation could be better than a Z5 / Z6 segregation. But Nikon has to think about how to make those huge full frame S lenses look good on Zf...on Zfc even 50/1.8s looks funny.
2. Mid range: Z6iii. Pretty much guaranteed with Expeed7 processor. But I am wondering what sensor would it use. I doubt Nikon will use Sony's new 33mp because it is a bummer. Bad rolling shutter and only cropped 4k60p. This is not any better than what Z6ii can already offer. Also because Nikon does not have A9 / R3 like Sony, I believe they can offer Z6iii a sensor with much faster readout speed. Maybe not as good as stacked sensor design but maybe 1/60s. This is already much better than A7iv.
3. High-end: Z8, Pretty much guaranteed with Expeed7 processor and a 'pro' body design. I guess it will use Sony's 60mp sensor like D850.