Are you willing to pay say $3,000 for the body…because that's probably what it would cost. Sensor would be 19MP assuming it's a scaled down Z8/9 sensor which while adequate would be a marketing downer, and making a 24 or 30MP sensor is a whole new development of a back side illuminated fast sensor. Add in the better EVF, the Expeed 7…and sure, you could probably fit that in a Z7 sized body but anything smaller would mean a smaller battery and again…marketing downer. And all of that is assuming that a smaller body can handle the heat that the sensor and electronics generate. And then consider the market…how many people would buy this $3K body?
The Z7II is already smaller and lighter than a D500 is…and I would bet that a large portion of the decision to make the Z8 what it is instead of putting all of those Z9 guts into the Z7II body and calling that either the Z8 or Z7III has to do with either physical size limitations, battery capacity, or heat load.
Granted…the Z8 is bigger and heavier than the Z7II is…and there's a reason for that difference. The sensor is the same size although a BSI one might be thicker. Don't know if the Expeed 7 is significantly larger than the Expeed 6 but probably it is given the huge performance difference with the according heat load. Battery…anything smaller than the Z7II would use the En-EL25 like the other crop sensor Z bodies…and it's less than half the capacity of the EN-EL15C that the Z7/8 use…so another marketing downer there.
I'm not going to say never because I don't know Nikon's thoughts…but there are just too many roadblocks both technical and economical to make a crop sensor sized body with Z8/9 guts in it…and you know what…the crop sensor body with Z9 features already exists, just put the Z8 in DX mode…or if you want light get a Z7II and put it in DX mode, but you won't get Z9 performance.