The castle zones would be great to recycle for content too. There's a lot of space in them that isn't really used for much, it's just sort of there; why not take advantage of that design space for dungeon delving content? Garlaige Citadel [S] could be pretty cool too, it has a lot of distinct sections that could have themes or purposes or monster families.

It might not be a great fit for Abyssea, story-wise, but at this point we have so many parallel dimensions and split timelines and unexplored alternate realities that it would be pretty easy to come up with somewhere it would fit. Maybe this is a small piece of Tartarus or Valhalla, or the home of the mysterious creators of the caturae?