So many possibilities. (2.4 spoiler ahead)

Not sure if it's time for Zodiark yet, Zodiark seems to be a major enemy in this story, more important than Bahamut (considering Bahamut was used by the Acians to cause enough destruction to summon Zodiark - unless I'm wrong), 2.4 also hints the revival of Midgardsormr, who once revived I guess will bridge the current story to the Dravania - Ishgard war for Heavensward, I wouldn't discard Odin as a regular primal battle either. There's also Omega... not a primal but could serve as a major battle before Heavensward.