My guess is that the final antagonist will actually be Venat. Mostly for the sake of the namesake but also because I can easily see Zenos and Fandaniel being dealt with right around the same time as Vauthry during Shadowbringers only for Venat to become a thorn in our character's side.

Venat's counterpart wasn't a benevolent individual in Final Fantasy XII and irreversibly damaged Ivalice for the worse as a consequence of her scheming. Given that the same plot point has been carried over but with the Sundering...I think it'd be pretty disappointing if there were no actual twists and the end boss is Zodiark or Zenos.