I might be off topic but i had an idea of a dungeon recently.

The entrance would be at the black brush station in central thanalan and the whole dungeon would be in the phantom train and you have to reach the first wagon of the train and defeat the phantom train (maybe make use of our phoenix downs we get in the later quests to do more damage to the train that would be cool). I don't have any idea how they could put it in the story or anything, it was an idea like many that i came up with