More likely it would follow Dissidia Duodecium and just have FFXIV only represented by Cosmos's side like FFXI was (only having two Cosmos characters - Shantotto and Prishe) - personally I would have thought Derplander would have made a better choice for FFXIV representative but, there you are ;).
Also I remember reading an article about the development of the first Dissidia and how Nomura and Kitase decided that each character would be unique in playstyle, abilities and appearance, so that makes it even less likely we'd see Gaius, Nael or another Garlean as an enemy, given the fact they would just end up looking too much like Gabranth from FFXII who is already in Dissidia (this was further proven why FFIX's Vivi has not appeared in it despite his popularity - there is already a 'BLM' character in Dissidia in the form of FFXI's Shantotto).
Either way, I guess we'll see soon enough. :)
