I have been thinking about it arriving next year and how it should be differentiated from the current lineup. First, it should be more than 45 mp. The Z7, Z7ii and Z9 already cover that. So it should be in the range of 70 to 100 mp. Second, it should be focused on portrait, landscape and studio work (traditionally medium format territory). Notice that medium format cameras don't now feel, and have never felt, the need to copy the old 35mm film format. Rather, they tended to follow either the square format or the old 5x4 format. I would like to see Nikon "freed" also from the 35 mm film format in the Z8. Third, I don't know the circle of light projected by current Z lenses but it must be large enough to cover the long end of 35mm so therefore it must be good enough to cover a width of 36mm. Why then limit the sensor to a height of 24mm? The circle of light must be good enough for a 36mm square sensor, even if there is some falloff in the corners. West End Photo suggested this on NR many years ago. However, I suggest something a bit different. I suggest Nikon work with a sensor format that is the old 5:4 format and simply fit the Z8 with as large a sensor in that format as they can fit into a Z mount body. Call is "Medium Format Lite." Shorten the width, increase the height and simply make the entire sensor larger with as many megapixels as currently possible. Make the Z8 a cheap competitor to the Fuji GFX bodies and the Hasselblad X2D with the wonderful advantage of not having to buy any new lenses. This could create a new category and sell well to both current Z body users as well as attract people away from Canon and Sony. Of course, my idea is far too late. Nikon has already made its choices and created prototypes which must be out in the wild undergoing "real world" testing right now. The odds must be over 95% that it has a standard 36 x 24 mm size sensor with the only question being how many megapixels on it and what level of image quality they are able to obtain from that sensor.
As Thom etc. have said they are in a bit of a tough spot WRT the Z8. Lots of folks (myself included) want a gripless Z9, but that would of course take away from Z9 sales. A higher MP model may make the most sense but then they still would likely have no true "mirrorless D850" since I imagine it would not be quite as fast as the D850 is.
As for the mythical Z8, I think most of us hope for a mirrorless D850. I have not tried a Z9, but Nikon claims it is the most SLR-like viewfinder in an EVF. The EVF is the main reason I do not like mirrorless, though I do own one Sony A74 in order to use their fantastic 200-600 lens. I got it for the lens, not for the camera, as I much prefer the look of an SLR viewfinder and still use two Nikon D850 bodies for everything except wildlife.
Is it the weight factor perhaps? To me not that significant as I use a grip on my Z6ll but certainly makes a difference.
I find the grip reassuring in that, possibly due to the size of my hands, the extra surface feels more secure with a grip (and for the sort of photographs I take, the extra battery power helps considerably!)
Regarding a pseudo-medium format, I can't imagine NIKON going that route with this generation of 'Z' series - perhaps a new series, the 'Y'...
You have probably seen the new post (linked above) on the main page of this site that says it might finally be coming Spring 2023. It is just a rumor (and there is no indication it is from a proven source, so I would take everything as hearsay). They do say (if true) they intend to compete with Sony A7R5 which I think they would need to, because based on specs that seems like the ideal mirrorless body. However I find the form factor of Sony cameras lacking, even though they increased the grip depth on the latest models. I know about the form because I have a Sony A74 that I got to use with their excellent 200-600 lens (I use Nikon D850 with my other three lenses). I like the lens, though it's a bit heavy, but I really don't like the camera and am likely going to sell the setup and get a Nikon 80-400 instead (I just got a rental to try out).
If the rumored Z8 can have specs similar to an A7R5 but a better body, it would be a real winner. By "better body" I mean more substantial than a Z7ii but not as bulky as the gripped Z9. At the moment I am happy with my two D850's and have no intention of going mirrorless, but I think a lot of other people would switch if they do the Z8 right.
I also don't see why that would be anything more than a Z7iii. There doesn't need to be a 8-series camera and the 7-series already fills in the price point of the D800 series and Canon 5-series. Putting out a high end Z7iii then a slightly higher Z7iv rebadged as a Z8 doesn't make sense at all, so if a Z8 is coming, I would expect it to be quite unique and filling a role that hasn't been filled before.
I agree with @MrFotoFool - I think we would have some more concrete evidence of the Z8's existance by now if the announcement was going to happen in the next couple of months. I'm not sure we have anything other than wishful thinking.
Having just rented and tested a Z7ii, I noticed one huge flaw that makes it (in my opinion) unuseable for wildlife. There is a two second startup lag from sleep mode which means you will miss shots (I missed a coyote waiting for the camera to wake up). My brother has two Z9's which are of course better, but he says even it has a small lag. I just put up a Sony A74 for sale since I don't love Sony cameras, BUT at least it has no lag. If Nikon expects to attract wildlife shooters, they will have to fix this in the upcoming Z7iii or Z8 or whatever it is called.