To write a good Samus Aran slave girl fanfiction, it's important to have a clear plot. Maybe she's on a quest for freedom or revenge. Outline the major events in the story before starting to write. Also, don't forget about the setting. Describe the world where she's enslaved in detail, whether it's a spaceship, a planet, or an underground lair. This helps the reader visualize the story better.
Develop interesting relationships in the fanfiction. If she's a slave girl, who are the people she interacts with? Are there other slaves she befriends? Or perhaps a sympathetic captor? These relationships can drive the story forward. Also, add some action sequences that show off Samus' skills, even in her enslaved state. This keeps the story exciting and true to her character.