Hi,
It basically boils down to two choices. The Sony 100 MP 44x33mm crop sensor or the physically larger Sony 150 MP sensor. The 50 MP one used in the 645z has reached the end of its production life as evidenced by the removal from the Sony component catalog.
So, users of the part will have been given a Last Time Buy and when those parts are used up, well that is all folks. I know how this all plays out. I have been on both sides of end of life parts for decades.
Fuji has stated that the 100 MP part requires IBIS. And, I have no reason to doubt that. So, there is one very likely new feature in a Pentax 645 follow on model. But they might well use the 150 MP part as they have lenses to cover that. Or, even offer both. We shall just have to wait and see.
Me, I'd be happy with either. Save me from eventually buying a Fuji 100 MP about a year from now. But, using most of my P645 lenses still via an adapter. And the 67 lenses via two adapters.
In the meantime, my antique 645D is doing fine and making me money.
Stan