Bob,
I think you're missing the point here with all this nomenclature talk. I realize most people are caught up on this nomenclature, BS, but many of us don't give a rat's a$$ about what it's called.
As I've said before, many of us want as much of the Z9 goodness as current technology allows to be squeezed into a D850/D500 sized body. I have a Z9 and I'd like to have a second body, sans grip (grip optional) that I can have with me as I usually did with my D850/D500 combo. We'd like the same ergo's and control/button layout as the Z9 so one can move easily between the two bodies, just as with the previous pair. That is never going to happen with the smaller Z6/7 bodies, there is not enough space to do that. A larger body and creative design might allow space for a new battery that is more powerful than the EN-EL15 series, too...something smaller than the EL18 series.
If push comes to shove, I'll end up buying another Z9 before buying a Z6/Z7 sized FX body...and spending an extra $2K isn't going to change my standard of living in any meaningful over the balance of my life, is it. Well, it might make me stronger by carrying that extra weight around. :)
Also, with regards to a second body being DX, as the D500 was...the only reason I chose the D500 over the D850 as the second body was for the higher frame rate, greater AF coverage and, to a lesser extent, the lower price. At this point, I have zero interest in a DX body as a second body...rather have two FX bodies, I can make either one of them DX at the touch of a button, and the frame rate/AF coverage advantages no longer apply.
In my case, is there a place for a smaller form factor body...sure, as a standalone, two lens, DX travel kit. I almost pulled the trigger on a Z50 during the recent sale, but I'm holding out for a new model with improved processor...hopefully they'll move the Expeed 7 through the full lineup over the next year. I still have a D3200, 12-24 and 18-300 kit that suits me just fine for a minimal travel kit. Still have a few F primes if need be, too.
Cheers!