So an idea I have for a voyage/enemy would be related to the Sirens. The riddle you get if you choose to hunt for them would only mention the area of the map where the Sirens are roughly located. When you get close, you start to hear the siren's song. From here one of two secanarios could happen.
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You start hunting for a buried chest digging it up will get you the Siren's chest.
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You run into a Siren. Defeat her and receive her skull.
Now both of these things would be worth a bunch, but they are very loud and alert other pirates nearby that you have collected Siren loot. This will make you want to sell your bounty quickly otherwise someone else will sell it for you.
It could also be possible to incorporate this type of voyage into the merchant quest line by having a "Siren's beast" that would have similar attributes to the chest or skull.
