I mean, bait aside, this is kind of a "blah" statement, no? There was really nothing to "call", as it was very obviously going to be a thing. Final boss? Probably not.

Of the Ivalice/Bozja "Lucavi" creatures, Zeromus is one we have not seen or heard from. He is also loosely associated with one of the Ascians via FF12's zodiac.

It's also would not be unheard for Zenos to fuse with Zodiark somehow like he did Shinryu and/or become Zeromus.

More importantly and unusually compared to older expansions is that the main Zodiark story will wrap up in 6.0; Anything 6.1 onwards will be an entirely "new arc".