i cant remember which FF series it was i think it was mystic quest that had cecil travel to the moon the content was much but it was fun. personally i like to see a lost underground kingdom like in the older FF series. but if you also add sea like under water ruins of some ancient race. also floating island and maybe a twist like in FF11's Past you can have up to 8 content areas, present land (current land), underground, sea, sky and past land, underground, sea, sky, which each can be expansion of its own....

for dungeons i think each current nations land (La Noscea, Thanalan, The Black Shroud) should have a 20s & 30s dungeon... having a 10s dungeon might have some worth but excluding a 10s dungeon will help push to achieve and feel accomplished when they are finally able to reach a small goal like being able to go to their first dungeon. then Coerthas should have two 40s dungeons. well you want adventurers to work toward a central nation like Ishgard then adventurer out away from any nation like to Mor Dhona. then i think the first expansion would be Underground, then a sunken ancient sea ruins, then a floating sky island kingdom that might send you to the moon for a dungeon.