First of all i wanna say that the game looks amazing and the devs have done a decent job of adding content to the game since release but that being said it can still feel repetitive once you get to max reputation so here is my suggestion to spice up the game.
1:- Add a human NPC nation who is enemy of the pirates and would have couple of non-capturable cities around the map preferably in each corner which we can raid for loot, they would have human npcs and ships of there own. They would also fight us for control of capturable forts around the map which would give rewards to the owner nation or something similar along those lines.
2:- Different types of Human NPC ships sailing around the map doing merchant trades between there earlier mentioned cities/forts or hunting down pirates (us).
Adding these two things to the game would make it feel alot more alive and would make the game so much more exciting.
Now i know these two things might be hard enough to implement but if you guys(devs) are really feeling ambitious you could make multiple nations with there own economy and governament. They would also fight each other and us for control of forts/cities which would give rewards for the nation controlling them.