I don't really believe Blizzard is intent on more classes.
If they do, I think we would need to come to a point in time where it would make sense to start up another prediction. But right now, considering all their efforts are directed into Evokers and Dragonflight, it's a complete wait and see.
We don't even know at this point whether they even want more classes in the future.
I would say right now, Tinker is probably the only legitimately discussable concept that has any realistic chance of being made, and everything else is literally based on wild speculation considering there's absolutely nothing indicating any other class concept becoming playable. None whatsoever.
Tinker is only kept open by the mere fact that Undermine is known and not yet presented. As for your predictions of Zin Azshari in the past, I personally do not regard that as being a realistic prediction because you directly drew from a historic source which is as good as wildly speculating that they're going to do a War of the Ancients or War of the Shifting Sands expansion in the future. We don't know if they ever would touch on those subjects, at all.
Undermine is a tangible location that has future potential, and a missing major character who has direct ties to that location and to the main story.