Many of the older photographers on this form were around for the Post-Film era when we transitioned into DSLR's. This pre-pre anouncement for the Nikon Z9 points towards the Post-DSLR era or at least post D5/6 era.
I think several conclusions about the camera can be drawn based on the scant information provided. Obviously from the photos the camera will have a built-in vertical grip rather than a removable one. I would expect the build quality to be robust with excellent weather sealing and excellent ergonomics. For sure the best build quality of any of the mirrorless cameras they have released and perhaps better than any of the DSLR's. Nikon has a stellar reputation for build quality so I think this is a given.
The sensor regardless of who it comes from will be of the "Stacked" architecture, with my limited understand of how the sensors are produced let me expand on what this will provide to underwater photographers. The key is speed not yet seen in other sensors without the expense of high heat, loss of high resolution, or loss of dynamic range. I have the Sony A1 in hand and the specs are possible because of this now third generation stacked sensor used in the camera. The camera has 8K video and because of the sensor design it can be run for close to two hours pretty much until the care fills up without over heating like the Canon R5 because of the sensor design. the speed with which the information moves from the sensor to the card (Sony has also introduced new high speed cards) is remarkable. It allows you to shoot with the mechanical shutter and sync your flash up to 1/400th sec. full frame or 1/500th sec. if you switch into APS-C mode, often used by sports photographers. In electronic shutter you can sync the flash up to 1/200th sec. These are things that no other camera can do and it is a result of the stacked sensor. Regardless of which sensor Nikon chooses for the Z9 I would expect to see similar or better results. In mirrorless cameras auto focus is off the sensor and with the Sony A1 the camera can refocus like 120 times a second. The electronic shutter can shoot 30 FPS, so fast that not all of the Sony lenses can keep up with the AF speed. For those shooting sports like the Olympics the A1 and Z9 will surly add value because they both will have sophisticated wireless data transfer and unlimited shooting in Jpeg. The Associated Press has already adopted Sony as their camera provider for its speed and ability to shoot silent in electronic shutter among other things. I am sure that the Nikon Z9 will also grab a large piece of that market. For all the Nikon faithful I would hope to see Nikon continue with the base ISO of 64 like in the Z7 II rather than the ISO 100 like Sony cameras. I find the slight increase in dynamic range at 64 much more useful than ISO-one billion.
I agree with Adam that a native auto focus fisheye would be well received by all underwater shooters moving to mirrorless and I sincerely hope that Nikon provides one. The only company that has been consistent in this regard was Olympus now OM. My first DSLR was the 5MP Olympus E-1 an excellent camera for its time. This was Olympus first entry back into the camera market after film and Olympus introduced an 8mm fisheye as one of the first six lenses.
Regarding Canon's sensor business the same rumor mills that brought use the Nikon Z9 are now speculating that Canon will release a new ESO R1 flagship camera to compete with Nikon Z9 and Sony A1. This camera is rumored to introduce a high MP global shutter of Canon's design. If this is true the restraints of shutter sync speeds could pretty much be removed because the "global" sensor collects all of the light at the same time rather than from top to bottom like most current DSLR and mirrorless full frame cameras.
Final word is that you are dreaming if you think the Nikon Z9 will come in at $6000.00. To hit that target price they would need to take shortcuts like a lower res EVF in the 5.7M dot range rather than the 9.44m dot range of the Sony A1. It would be a pity if Nikon goes this route like they did with the 3.6m dot EVF in the Z7 II rather than the 5.7m dot EVF's used in cameras at smilier price points.