Yes, the rumors of the Z8 have been going around for a while, and it's still no show, although it's out of my price range.
I would love to know what kind of price this is going to be sold for. They compare it to the Canon EOS R7 which has an MSRP of $1399. If the Z90 comes out with the rumored specs and around that price, I might be interested. It would effectively give me a 1.5 teleconverter with no image quality loss or F-stop reduction. Not sure how the autofocus for wildlife would compare with the Z5 I have now. My assumption would be that it's an upgrade, and the increase in fps would be nice as well. It would be interesting as well to see how the 33 MP crop frame sensor performs in comparison to my 24MP full frame. I would guess slightly better resolution and slightly worse low light performance.
It would be nice to turn my 200-500 F5.6 into a 350-750 F5.6. This lens plays nicely using the FTZ adapter, and has very good resolution. AF is a little slower than I'd like, but the alternative is a very expensive native Z mount. It would also extend my 24-70 out to 105, although that is a much less useful combo. When I'm out with my 24-70 mounted, I go wide angle as often as I do short tele. I would be better served to get a longer zoom. A 70-200 would be the ticket, although right now I would pretty much have to go expensive, a super zoom like the 24-200 or an affordable F mount. Right now I think I'm better off spending my time on getting better with what I currently have.