Kain actually already is in Dissidia (he first appeared in the second PSP title Dissidia Duodecium, and was added into Dissidia NT as a DLC character as I understand it), so that automatically rules Estinien out, as one thing about Dissidia is the mainline titles try to have each fighter unique, so having two DRGs in there is just contradictory.

Personally I'd love to see Ardbert and his Source-counterpart/player character standin 'Derplander' (maybe have them the same character shifting between 'Source' and 'First' versions?), as he/they are the real main character for FFXIV and are the closest representation to the player's character being able to appear in Dissidia. Maybe even him/them class change between GLA/ARC/WAR/DRG/MNK/SAM/DRK/PLD based on abilities, but WAR being the 'default'?

Ignoring the crazy stalker being already in Dissidia NT, I'd still also love to see Gaius or Nael appear as a villian in Dissidia too, although again given Gabranth already is in the game, and there is also an excess of 'heavily armoured' villians, maybe something different for a change could appear... like an Ascian...