This is a suggestion that was made countless times, but it is my turn to take a hit at it.
Every month, a powerful sea monster spawn in a random port, everyone can attack it.
The boss is global, meaning the damage dealt to it is added from other player attacks.
Attacking it cost 25 turns.
At the end of the event, the top 10 damage dealers in the guild with the most damage dealt will earn rewards. Voodoo, money, gold, credits, etc.
At the end of the event, the top 10 solo damage dealers will earn rewards. Voodoo, money, gold, credits, etc.