Okay… This idea is Waaaaay out there. Okay, when you became a Legend "was" the first time you really encountered "ghosts"... Now it wouldn't be a far flung idea to think that, "if there are ghost pirates, there are also ghost monsters of the deep, long forgotten but still thirsty for the taste of pirate".
So the missions are largely taking down huge historical horrors of the sea, but only A pirate Legend can see them due to their exposure to the Legendary Hideout and The Pirate Lord.
Each mission would revolve around the naming of an Island or appearance... So if you got the mission "Defeat the Kraken of Legends" you'd go to Krakens fall... On approach the island would split, the physical Island would still be there, but out of the land would emerge a HUGE Ghost Kraken.
These Ghost monstrosities could only damage a ship on that current quest and the monster would appear invisible to any non-Legend. So to any other "non-legend" player it would look like a Legend had gone mad and was fighting thin air.
Snake Island would morph into a Huge leviathan... For some comedy, chicken Isle would spawn a Gigantic ghost chicken, A huge evil ghost ship would emerge from Crooked Masts, Shipwreck bay would look like a Giant Sarlacc (from star wars)….
The initial quests would be the smaller isles, twin groves, defeat 2 Megalodons at the same time, Rapier Cay could be a flying fire breathing Phoenix and the very, very final boss... has to be.... The isle of Last words! A fight with the pirate Grim Reaper. The surrounding rocks are really just points to his crown.
