I'm sure this has been suggested before, but never hurts to build on each others ideas if it has. So without further ado!
A invasion of a royal navy of some kind or something posh and rich with wealth!
After all... wouldn't some corrupt/pure king of some kind eventually find out about the trading going on in the sea of thieves and send ships to investigate? This could even be tied into why the merchant alliance is working with pirates in the first place, maybe they went rouge and found pirates to be more profitable and now some powerful person in charge is angry and wants justice.
Thus you get a full scale invasion...
Phase 1. Enemy ships would maybe bring supplies to the unused forts in the sea of thieves to claim them as their own. So maybe you have a Bilge Rats event were you try to take out merchant ships along with there escort. Escorts would be War ships, a 6 man ship rocking 6 below-deck cannons (See phase 3 for details on how ship would be unlocked)
Phase 2. They succeed with numbers and now forts must be taken back from them using ground forces, since walls would be reinforced (maybe stone with many cannons) and enemy units with be barricaded inside. Units would include a mix of knights and riflemen. These enemy units would protect these forts with the idea of controlling the sea of thieves and ultimately claiming it for there own.
Phase 3. Once the enemy is removed from the sea of thieves, we would follow them back to a massive port (new area on map) which would be accessible ONLY for pirate legends, since the King refuses to deal with lowly s**m. We would have the king ready to hang only to have him threaten us with an even bigger threat if he perishes. So we would settle a peace agreement that would allow legends to do business in the port (exclusive shops and voyages which could make use with new seaports). This would then bring me to a new game mode and exclusive new weapons for Legends ONLY.
PvP arena game mode for 6 man War Ships
Form a permanent alliance with 6 of your legend friends and duel other alliances to the death. This would bring out a leaderboard AND exclusive alternate weapons for Legends such as
A double barreled Blunderbuss - Has two shots, but longer reload time
Unscoped Eye of Reach/Rifle - has same range as Eye of Reach, BUT gives you peripheral vision and faster reload.
Duckfoot Pistol - a multishot pistol with long reload time
Hand Cannon - A mobile mini cannon that fires ANY cannon balls you have on hand, Long reload time, short range, and hard recoil.
Finally, a ton of melee weapons with different ups and downs
These weapons of course would be legendary in there own right and should not be easily accessible but EARNED through the pvp war ship arena/new voyages/athenas.
Well thats my idea, I encourage everyone to please build on it and let me know what you think!
Thanks!