26mm sounds like a nice focal length for urban photography. Sometimes 24 is a bit too wide, and 28 too long.
I put a pre-order in for Z9 - I expect it will probably be next year as I have no sort of priority at all.
Then the lens should be like f2.8 and made as small as possible. Am looking forward to seeing your Z9 in Tokyo next year.
I fully expect the 600/4 to also have a built-in TC, so you would buy a lens depending on whether you need 400-560mm range, or 600-840mm range.
The updated roadmap also shows another 400mm lens and an 800mm lens. The new 400mm is short which suggests it is a PF lens. The top is the same size as the 58mm Noct so probably has 95mm filter size. This would allow a maximum aperture of f/4.5. With a 1.4x TC it would give a 560/6.3. The lens/TC combination brackets the 500/5.6 PF, I'm no sure why they didn't just make a Z version of the 500PF as it is a very popular lens.
The 800mm is only the same length as the Z 400/2.8 so is also a PF lens. If the aperture is f/5.6 the top should be the same size as the 400/2.8, but it appears a bit smaller. Maybe the aperture is f/6.3, assuming the profile is accurate.
Not long after Nikon had released the latest versions of F-mount 600/4.0 and 400/2.8 employing FL elements, both Sony and Canon released the rivaling models which were roughly 1kg lighter than the Nikon ones. Canon released RF mount versions apparently using the same optics. Combined with lighter mirrorless bodies like A1, A9 or R3, their advantages in weight was significant.
The 400/2.8 with the integrated 1.4x TC could be a nice counter punch to Sony and Canon, even though Z9 is much heavier than A1 or R3.
I also suspect that the smaller 400mm would be a PF lens. The 600mm and 800mm with integrated TCs would be interesting. The TC for the 800mm could be a 1.25x one?