i have made a post skimming over this idea along with a few others but i believe this needs more attention and clarification.
a new island to the NW area of the map, right at the edge by the Blood Red Sea. Have it be a beautiful looking island to match the nearby ones but with a giant ancient temple like structure on top where the entrance is at the top. To find this island you would have to complete many riddles and puzzles. You would have to stop your boat by the front of the island due to a big wall all around the whole temple, and a locked gate which requires a random key. The key can only be gained by killing a skeleton Captain who spawns outside of the wall or perhaps on his own sloop near the island and then you get a map to a chest with the key in it. Get the key, go back and unlock the gate for only just your crew and it will load you into a private instance of the raid.
From there, you will fight through rooms skeletons, skeleton lords, giant spiders,ghouls and witches/necromancers as well as having puzzle rooms mixed in as you descend down the temple until you break into the treasure room. You'll collect like 10,000 gold that is on the ground and from collector's chests, then pull a lever to open a back door which leads outside and back to the gate where you'll carry all of the chests (as there should be a lot of chests to turn in), but gotta remember. Once you leave the raid's instance, you're back in the open and another ship could've sunk yours while raiding and is waiting for you to come out...
i feel like there should also be a requirement of at least 3 people to complete this as some of the puzzles may require at least 3 people but because of the challange i would recommend 4.
before every major bossfight there would be a room where you can refill ammo and food