Whether the rebate does indicate or not, a Z6 III is overdue (as already mentioned above)
The main deficiency of the Z6(II) is not the sensor but the AF speed and accuracy and subject recognition. I then have chosen the Z6 over the Z7 because I thought its 24MP is more suitable for use with old manual lenses and it offers advantages for High ISO so I see no need for more megapixel.
A Z6III should have change Z6 lines horrible user interface to be more Z8-like, but I like the size of the Z6 and the way it feels in my hand.
I agree they need to work on the algorithms. The Zf already can do very well in autofocusing on people subjects and porting the bird and animal code from the Z9 should be straightforward. The Zf is of the same price class as a Z6 III would likely be. The brass dials etc. and forced thin chassis probably lead to higher costs and a Z6 III with Zf internals might cost a few hundred euros less than the Zf, if they use the same sensor. If there is a new sensor then the Z6 III might cost a few hundred more than the Zf.
How would you change the Z6's user interface? To me it's not that different from the other cameras in the Z line. I think what would be good to have is one more function button and more customization ability (some of which the Zf already has). I think the smallish body size probably is favoured by many and there is not infinite space for a lot more physical controls. Although adjusting settings is a bit clumsy on the Z6 II, due to many things forcing a menu dive and there are multiple menus (main menu system, My Menu, and i menu, plus the info screen; very complex).
What I'd doubt is, whether a Z7 III (and yet another 45 MP camera - there are already three) does make any sense (now that the rumors of Z8 having 60 MP have proved to be false)
The Z7 chassis is smaller and lighter than that of the Z8 or Z9, and there is a bit more dynamic range due to the slower read time of the sensor. I think what the Z7 III could get is a bump in resolution, hopefully at least 2x the pixel count of the current version so it'll be a noticeable difference and people will turn heads. Of course one can question what is the benefit of higher resolution when people post images on FullHD-res instagram and view it as a few degrees of their visual field. But then there are those who print large and want to crop as well. I think a Z7 III with 100 MP would turn some heads of current medium format users, and if Nikon launch it with a set of PC Z Nikkors, there would be some novelty with those as well. Make them resolve 100 MP cleanly. ;-)
Sony announced a global shutter and there will be a Canon EOS R1 with rumors also assigning global shutter capability. So Nikon will be forced to react. But I am 100% sure that a Z6III would NOT be the first Nikon offering global shutter feature. Probably it will come in a Z9II (which will be needed as a reaction to R1 somehow) but I would not expect that before the end of the year 2024 or even more likely not before 2025
There are certainly a lot of rumour site posts but I think they're more like wishes that people express rather than fact-based. They do a really poor checking of facts if they do any checking at all.
Canon uses dual pixel AF and it means they need two photosites for each image pixel. If they want to compete for the high resolution high speed market, they need to put at least 45 or 50 MP into the R1. In practice then they need a 90 or 100 million photosite sensor with simultaneous read of the whole image including AF data, if it is to have global shutter. I just don't see this as realistic. Sony's GS camera only does 24 MP, not 90 or 100 MP. Is Canon so far ahead in sensor technology that they can read four times the amount of data simultaneously?
In my opinion global shutter has some value in photojournalism, video and related fields but since the image quality is likely to be somewhat inferior (reportedly Sony GS for industrial applications has more than twice the read noise, 1/2.4x the full well capacity per area compared to rolling shutter sensors by the same manufacturer) and cost is high, I don't see them becoming all common as general purpose cameras. The bird people will want higher resolution, and sports photographers have to submit their images quickly so they can't spend all eternity in picking the best frame out of a hundred thousand. Landscapes want higher image quality than either stacked or GS sensors can give.