Sadly I don't hold much hope seeing an antagonist for FFXIV appearing in Dissidia NT as FFXI never had antagonists representing it in the previous two Dissidia games either, they only featured protagonist npcs on Cosmos's side (Shantotto and Prishe).

It also doesn't help that the Garlean legatee like Gaius all look too much like FFXII's Gabranth who is already in Dissidia, so I'm liable to say one of the Ascians - probably Lahabrea or Elidibus. I'd prefer Gaius though, given he was the closest to an 'antagonist' for both 1.0 and ARR (he even appeared on ARR's cover for the US version after all). But again I don't see it happening alas.