I think a pirating MMORPG is the way to go. Set in a somewhat surreal monkey island esque world, where the aim is to explore the various islands and beyond. Treasure hunting, massive sea beasts, mermaids, ghost pirates, skeletons... certain environments or islands could be created procedurally to keep things fresh.

Buy bigger and better galleons, plunder entire towns, raid haunted castles.

It would be amazing.