I dreamt of a sea fort that players must defend against waves of PVE and PVP using a giant multi-operator cannon. I drew what I remembered using Sketchup.

Frustratingly, I didn't get to actually play the island in the dream, but the gist is that players must fend off waves of PVE with a giant cannon that requires multiple pirates working together to operate. There was some extra incentive for players to attack the island and for the defenders to sink player ships.
The cannon must be loaded first with a large-sized cannonball that behaves like a cargo item, followed by a gunpowder barrel. The gunpowder barrels are kept in storage below so that attacking ships cannot simply detonate the entire stockpile with cannon splash damage. Barrels are brought up to the cannon with an elevator. The cannon barrel must be angled downward to load it.
Using the cannon requires teamwork. The roles are roughly 1) loading and firing the cannon 2) adjusting the pitch of the cannon and 3) rotating the cannon turret and placing gunpowder barrels on the elevator. In my dream, for all of this work you get a tier 3 hole that fragments into a scattershot within the ship. Perhaps it could be stronger but you can't argue with dream logic.
I think this could be a fun player-activated world event. For example, this could be used to revitalize the siren song quest by adding the option to empower a siren skull that attracts skeleton ships to the turret with its siren song. Defend for long enough and you get a siren skull with increased value. Rare: feel free to steal any and/or all of this without providing money or credit, I just want to play the island of my dreams!