My guess is Merycidia and the "state of affairs" over there. If I had to wager a guess, I would say some faction from there is most likely going to be the new outside threat Eorzea faces. Think about it. Ages ago they got curb stomped by Allag, they have resentment over what Allag did to their continent. Allag was centered in Eorzea with primarily an Elezen population. Merycidia is mostly Hyur, Miqo, and various beast tribes we've never seen, along with some dragons of Baha/Tia's brood who could still also be showing affects of tempering. LOTS of opportunity to explore the racism that exists in the FF lore that only slightly gets touched upon, usually, or worked out very quickly.

I'd love to see a political and social expac, much like Heavensward again, but centered around this people and culture who basically have stewed in their anger at Eorzea for thousands of years. Maybe they send agents to try and politically destabilize our alliances here, and then 7.0 we go there to repay the favor.

14 for a long time has made this huge emphasis on togetherness and working out differences, I'd love to see a situation where we CAN'T do that. Where it's an actual aggressor that can't be reasoned with by the 'Power of Anime'(TM). Merycidia could be all that and more. A foe that no matter how hard we try and what we do over there, they are going to be determined to be a threat to us and ours, to Eorzea.

Plus so much room for lore expansion there!