I wasn't thrilled with Zenos as the Dissidia villain, but as it seems like he's going to be around for a while, he may end up being the longest running main antagonist FFXIV has in the end so in a way he may be fitting even if he's a boring character.

Gaius was brought back but will probalby be more of an ally than enemy now. Lahabrea has been dead for some time. Emet-Selch was fantastic but left us too soon. Thordan and Nidhogg were one offs. Nael was important to 1.0, and then a puppet of Bahamut in 2nd coil but has been outside of the fray for a long time. Elidibus would probably be the most fitting thematically at the moment but he hasn't been shown to be a combatant at all, and the way Zenos' story has been going he might end up as the final boss of the overall Ascian/Empire/Zodiark plot pulling a Kefka to usurp the plans of the other villains for his own self centered goals.

That's sort of the issue with FFXIV in general, most FF games are single player one and done games that have an easily identifiable main villain.