Sacred Pool. It gives You some usage for the Neutral character, some horses and mules to carry more objects, and really great and awesome Spells to bug Your friends.
You may also consider the Firelands. This expansion is really dangerous, so the game will be much harder. You will watch Your every step . Plus, it contains the Terrain cards. If You will proxy some homebrew Terrain cards, You will be able to change Your main board into anything You want. For example Disneyland or Ravenloft.
For the board expansion I suggest the Highland. Unlike the Dungeon, in the Highlands You will have some hard times guessing what the top card of its deck is.