There is two kind of cameras that the DX range lacks at the moment :
the Z30 and Z50 are more or less filling for the previous D3x00 and D5x00 range.
We're still missing the replacement for the D7x00 and the D500's successor.
I feel like a D7x00 successor can't be priced higher than $1500. The competition is pretty fierce in that range (Fuji X-S20 - $1300 ; Sony A6700 - $1400 ; Canon EOS R7 - $1500) and pricing it higher would be a disservice.
For specs I would actually see Expeed 7 + the current 21MP DX sensor that we have. The improved AF and video performance (maybe 10bit 4.2.2 4K30 no crop + 10bit 4.2.2 4K60 with a 1.5x crop) given with the Expeed 7 + a larger, better built body to match the one from the D7200 (bit taller and wider than the current Z50, dual SD slots, EN-EL15 type battery, IBIS, 3.69M dot EVF, fully articulated screen, 15fps mechanical shutter, 20fps electronic (with the drawback of heavier rolling shutter, like the Canon R7))
This could be called the Z70.
Second camera missing is the D500 replacement, and when the D500 was released, it was basically the best that Nikon could offer in a DX body : new BSI sensor, flagship autofocus.
I would expect no less for its mirrorless replacement, however I'm not convinced it would have a more than 21MP sensor. Maybe 26 if we push it, but I don't see NIkon making a higher resolution DX sensor, as they're reliant on Sony manufacturing processes, and Sony makes either 21MP (for Nikon), 24MP (for Sony), 26MP (for Sony and Fuji) and 40MP (for Fuji). Unless they make a stacked version of the 40MP sensor, using the process node that Sony uses for that small of a pixel pitch, then it would be under 30MP for sure.
And yes, it needs to be stacked if you want to have a similar AF performance to the Z9, which is what those cameras are about. Maybe give it 30fps shotting, surely the smaller sensor would be a little faster and would be able to push 30 instead of the 20 that the Z9/Z8/Z6iii does.
This could be called the Z90, and I could very well see Nikon selling it at about the same price they sold the D500, so under the Z6iii's pricing (wouldn't make sense to have them at the same price, the Z6iii would sell a lot more than the Z90 if that was the case).
So probably something like $2000 or $2200.
Any higher than that and you not only get the competition from the Z6iii, but also from other cropped sensor stacked cameras like the Fujifilm X-H2S ($2500) or the OMS OM-1ii ($2300). Nikon doesn't really have the lenses to compete with neither of those systems in the DX range, it only has better telephoto primes (and even then, it's not like Fuji and OMS are lacking on that front)