As much as I like mirrorless, sometimes I wish Nikon would implement more of that tech back into DSLRs. For example Nikon could make more AF-P FX lenses like a new 24-85mm AF-P VR and on my wishlist a 40mm 2.8 AF-P. They could also integrate OSPDAF into future DSLRs allowing for fast AF when shooting in live view. In fact I like that idea, because I get DSLR advantages for fast, blackout free C-AF, but when shooting from the hip or doing video using the back LCD, I get the same live view advantages due to OSPDAF. Also more F-mount AF-P lenses would make it easier integrating lenses between F and Z bodies.
I think a Z5 makes sense if its a cheaper, lower-spec model (either old FX sensor or APS-C) but I don’t think Nikon will try to make a lot of cheaper lenses for it (especially if it is APS-C). The APS-C DSLR lines never got a lot of love, and I would expect similar treatment if they moved to mirrorless. In my mind, going higher end is more likely, a “pro” Z8/9 that is closer to a D5 with built-in grip, large battery, etc.