*Spoilers* Who is the big bad?

The big bad is SeanCody

I don't think there is going to be a single "big bad" who has been behind it all.
The final antagonist for the current overall storyline is probably going to be Zenos after he has stolen Zodiark's power.

MMOs are better off without declaring a top big bad, so that they can keep them coming without disappointing anyone (see some wow players' reaction to the end of legion or even wotlk for those that weren't really into lore but in love with warcraft 3/darth vader 2.0).

So the real big bad ((truly final) for reals!) will be the mastermind behind the evil plot in the expansion where subs and store purchases drop to a point of non-profitability, and I like it that way. The least thing I want is FFXIV MSQ just having a definite end.

The real big bads are the server hamsters. Except they're not so much evil as mildly mischievous and delight in annoyance and inconvenience. Remember them next time you die to an AOE you swore you moved out of in time.