There could be a fanfiction where a group of pirates form an unlikely alliance with a mermaid. The mermaid helps them navigate through dangerous waters in exchange for their help in saving her underwater kingdom from an evil kraken. This would be full of adventure and unique interactions between different sea - dwelling creatures.
To write a good 'Sea of Thieves' fanfiction, you should focus on the setting. Describe the choppy seas, the creaking of the ship, and the salty air. Also, add conflict. It could be a rivalry between two pirate crews over a powerful magical artifact. And don't forget about dialogue. Make it authentic, like how pirates would really talk with lots of 'arrrs' and nautical terms.
It's likely about the adventures in the Sea of Thieves world. It might feature pirates, their quests for treasure, battles on the high seas, and the various strange and dangerous creatures that inhabit that world.