Mash together AC4: Black Flag and Shadow of Mordor/War to get a game where you play as a pirate in the Caribbean, fighting against colonising and slaveowning factions, divided into hierarchies like the Nemesis system. You could assassinate individual captains, raid their ships to lure them or weaken them, pit them against one another, threaten, bribe or convince them to be an agent for you, and so on. All the while managing your ship, building your crew, and traveling with a team of unique pirates and freed slaves and sailors and treasure hunters. Lots of side quests, exploration and hunting - honestly you could just rip all of the downtime from Black Flag, like diving, treasure maps etc. The interaction with the nemesis system could mostly replace any kind of gameplay/story division, because the story is the gameplay - its how you interact with the many enemies and systems in the Caribbean.

If you don't know what the Nemesis system is, you should definitely look it up. I think its the biggest innovation in triple A storytelling of the past decade, but its barely being used at all.