The thing about Zenos is that he's the only antagonist who's made things with the PC personal. Everyone else we've gone up against are just acting in their interests, and we go against them for ours (let's not get into ethical debates over the means and motives). Zenos, on the other hand, specifically allows the PC to survive and grow stronger for the sake of his own interests, and after his survived suicide does nothing but obsess over them. Further context notwithstanding it has to be Zenos because he won't stop until he gets what he wants and doesn't care what it takes. Even turning his homeland into a smoking waste doesn't seem to faze him, if the Endwalker teaser FMV is anything to go by.

As for why Zenos specifically, Fandaniel does question whether or not Emet-Selch "found a way" when Zenos mentions his dreams of the Final Days. Further, in the presentation it was said Fandaniel holds an important secret related to what's going on in Endwalker, and given the unanswered question above I'm inclined to believe it relates to that. What exactly that secret is I could not say.