Underwater would suit Leviathan, which is why I could see them taking that as their next step forward in content approach. They could incorporate the Sahagin HQ and Leviathan altogether in one neat package.

For the aerial approach, not entirely sure. Other than another massive Garlean airship stationed somewhere in the skies that we could invade and attempt to sabotage. Though, on that same note; I really want them to do something with the Floating City of Nym. That place just looks awesome.