I'm pretty sure if there's a ffxiv villain, it would be gaius...

I still hope, they'll choose Nael van/Eula Darnus because technically she and Y'shtola are both from 1.0 and Nael was the main villain back then. :3c
And I would be the happiest person if I could see Ser Zephirin on the side of the bad guys. Not because I want to beat him down but more because I'm one of those weirdos who really really really liked him in Heavensward.
They could bring Haurchefant as his good-guy-counterpart too.