My guess:
3.0 Heavensward = "Sky" - Ishgard arc + Dragonsong War and Ul'dah close.
4.0 Ala Mhigo = Eorzean Alliance (finally) supports the Ala Mhigan Resistance and battles to secure Ala Mhigo before the new Emperor puts his full force behind the assault on Eorzea. This would secure a fortified foothold to fight the Empire from and drives the Empire mostly out of Eorzea. Opens up more of the Shroud, and old Gridanian wounds and lore about the Autumn War and the nations' treatment of the Ala Mhigan refugees. And finally having most of Eorzea opens the next expansion up for...
5.0 = "Sea" Limsa Lominsa story arc that's been in the background since 1.0 days. With Ala Mhigo retaken, Garlemald confronts Eorzea on all fronts, and it's up to Limsa blockade to weather the storm while the rest of the Eorzean Alliance fights Garlemald on land on the Ala Mhigan front. This expansion would open up the continent of Meracydia to the south, Thavnair to the East, and the New World to the West. This is our "exiting Eorzea" expansion, but not quite pushing into Garlean controlled areas.
3.0 Heavensward = "Sky" - Ishgard arc + Dragonsong War and Ul'dah close.
4.0 Ala Mhigo = Eorzean Alliance (finally) supports the Ala Mhigan Resistance and battles to secure Ala Mhigo before the new Emperor puts his full force behind the assault on Eorzea. This would secure a fortified foothold to fight the Empire from and drives the Empire mostly out of Eorzea. Opens up more of the Shroud, and old Gridanian wounds and lore about the Autumn War and the nations' treatment of the Ala Mhigan refugees. And finally having most of Eorzea opens the next expansion up for...
5.0 = "Sea" Limsa Lominsa story arc that's been in the background since 1.0 days. With Ala Mhigo retaken, Garlemald confronts Eorzea on all fronts, and it's up to Limsa blockade to weather the storm while the rest of the Eorzean Alliance fights Garlemald on land on the Ala Mhigan front. This expansion would open up the continent of Meracydia to the south, Thavnair to the East, and the New World to the West. This is our "exiting Eorzea" expansion, but not quite pushing into Garlean controlled areas.
