There are so many requests for successors to various Nikon DSLR lines and I really think most (not all) of these people are setting themselves up for disappointment or at least are missing what Nikon has chosen to offer for these market segments.
I think we might see some kind of D500 successor, though I am at least a little skeptical. As for all the other ones, I think the "successors" are already on the market.
The Z30/50 are the cameras Nikon has aimed at the D3000 type buyers
The Z5 is the camera that Nikon has aimed at the D5000 series buyers.
The Z6/7 are what are aimed at the D7000 series buyers.
The Z8 is probably targeted towards people who would have bought a D800 series, and of course the Z9 towards the flagship buyers.
One might object that these models aren't directly equivalent to the DSLR lines, and I'd agree they're not but I don't think Nikon was ever aiming for direct equivalency. It's a new class of camera (mirrorless) with new standards for technology.
I also think that we're in a bit of weird place in the product cycle because while with the DSLRs there was usually a camera in each line series was roughly of the same generation of technology and shared tech, right now the Z line is split between the Z8/9 (and maybe we can put the Zf in there) which are share similar featuresets and performance levels (presuming that the Zf is as improved as it is hoped in terms of AF), the Z50/5/6ii/7ii are clearly a generation behind and the performance gap shows, so it's understandable if a D7500 user won't find the Z6ii to be a worthy replacement.
One might also object that the D7000/5000 series were DX cameras while the Z5/6/7 lines are FX, but I think this is another case where Nikon has decided to handle the mirrorless product series differently. They don't seem to think that a (physically) smaller sensor is a necessity or appropriate for these mid-level product lines anymore. One wonder if maybe they have a theory that with rise of cell-phones as competent cameras with very small sensors that a camera body past a certain price point need to have a full frame sensor to compete in the market.
Maybe that isn't the idea, but regardless I really don't think Nikon is going to put out a bunch of mid-tier DX cameras any longer. Back in the DSLR days they didn't have a bunch of different FX models along with their DX models. They had the flagship, the D800 series, and the D700s. Then they had all the enthusiast/entry level lines in DX. With the camera market smaller, - not larger - than it was before, I don't think it makes sense to expect them to have 5 or 6 different full frame series along with 4 or 5 DX series. I think they've shifted all but the entry level lines to full frame for whatever reason it is that they think that makes sense, and so the D7500 buyer should really be looking to whatever replaces the Z6ii as that is likely the closest thing to the D7500 we're going to see.
But, then my favorite lens would just form a low-rez circle.
I understand your concern. In fact, when I got my first Z body I chose a Z7ii instead of the less expensive Z6ii specifically because most of the lenses I owned were DX lenses, meaning that I really needed the extra pixels on the Z7ii sensor to get photos which were of a similar resolution to what you'd get on a DX camera body.
Again this is me reading in between the lines a bit, but I have to wonder if this was part of the reason they originally produced the Z6 and the Z7. Having two cameras like this which are essentially the same other than the sensor size isn't really something I can recall Nikon doing before - or if they did it mustn't have been very often. They didn't really ever treat megapixel count as a kind of "option" that you could choose between but as something sort of integral to their overall body designs - so having these two choices was a bit different. Yet initially, the Z6/7 were the only two options to launch the system, along with what was originally a very limited supply of native lenses, so it would make sense if these were originally intended to provide the kind of flexibility people would need if they wanted to move into the system while carrying over most of their F mount lenses.
I do think, though, that in the long run they don't intend to build the Z system in a way where the use of F-mount lenses is going to influence anything about how they design ongoing releases. I certainly think they want to maintain backwards compatibility with F mount as an overall option within the system, but I doubt that they're thinking of it in terms of keeping future designs in a perfect one-to-one relationship between product tiers across generations.
In other words, I think want to continue to support people being able to use their favorite F-mount lenses moving forward by selling their FTZ adapters, but I also don't think they're designing their "$1000 enthusiast line" or their "$2000 prosumer line" or their "$3000 pro line" around which 20 year old lenses you can use.
And frankly, I don't think they
ever really took the DX line all that seriously in terms of lenses. I say that as someone who for a long time only owned DX bodies and DX lenses. There are some DX lenses which are pretty popular and beloved, but at the same time I was always frustrated by how limited the DX lens lineup was. There are a lot of things that were never made specifically for DX bodies. For instance, they never made a super-telephoto lens for a DX body - not one! - and this in spite of the fact that DX bodies were very popular for use in the kinds of photography that use these focal lengths. Wildlife and sports shooters often used DX bodies, but were always adopting FX glass because they never made any of these focal lengths in DX. They also made only a very limited range of fast DX lenses. People loved the 35mm/f1.8G, and the 16-80/f2.8-4, and there were a few others that at least got to a wider aperture at the short end, but I don't think they made any fixed aperture fast zooms for DX, and most of the "bread and butter" lenses for DX were only available in significantly slower aperture ranges than for full frame. For instance, you couldn't get something equivalent to a 70-200f/2.8. Instead, you could get something like a variable aperture 18-135 which only got as wide as 3.5 and quickly got darker as you zoomed. With the crop factor, that 3.5 was really more like an f5 or 5.6, too.
So as someone who for a long time used DX and was always looking for the best DX stuff, I really have to say I don't think they ever treated the DX lineup all that seriously in terms of the lenses they provided. How much less of an emphasis are they going, then, to put on making those adapted lenses work in a one-to-one way with the next generation bodies they put out for these market segments?
