Rare, you are missing a treat with introducing these mercenary quests and yet its THE SAME QUESTS!?
Mercenary means sword for hire basically so why are we digging up treasure?
If there were NPC humans, for instance trade vessels going to and from ports, Mercenary voyages could be to escort them, or attack them.
Or as people have said previously, a bounty system, a crew going out of their way for pvp should get a bounty on their head.
If there were more humans in the world beside a lonely tavern keeper, actual pirate societies like Nassau once was, so much scope could happen in terms of content!
Events such as the “civilised world” trying to encroach on the Sea of Thieves, getting the players to band together to protect a certain outpost! Giving us reason to band together as pirates besides X marks the spot.