Dungeons; non-exhaustive, non-original
Humongous ship. Operative, haunted, sunken, any combination and degrees thereof. Half-sunken ship serving as a base of operations for a bunch of bay pirates? Ship with a troubled past: the only affordable way to sail.
Forest. Wet or dry? Lush or dead? Layered towards the sky or the sea? Open or mazy? Dangerous or deadly? Creatures, spirits or civilization? Hand-made structures? Ruins thereof? Completely untouched by civilization in forever?
Seafloor. Ruins? Very deep? Caverns?
Sailing. The ship is mostly under control, threats come outside. Sea creatures, weather, angry gods, pirates, hostile navy. Plot: Delivery, escape, decoy, search, warfare, exploration.
Cities. Politics, including uppity mayors, guild feuds and interracial conflicts. Thieves. Ship maintenance. Defense, sabotage.