The Rascal because it would give an update and minigame to the mineshaft which hasnt been updated in a while, they give you loot for finding it 3 times and its also cool for the lore because it could possibly be an ancient builder.
Rascal because the caves need a new thing. Their "mineshafts" and "dungeons" need a lot of work. The ocean doesn't need another addition, whereas the caves do.