Well, it could be a story full of sisterly love and adventures. Maybe Lucy and Mira would go on quests together, with Lucy's celestial spirit magic and Mira's take - over magic complementing each other. They could face powerful enemies and protect their guild mates as sisters.
In this fanfiction, their different personalities would also play a big role. Lucy is often more on the shy and cautious side at first, while Mira is this outgoing and friendly figure. Their relationship as adopted sisters could be about how they influence each other. Lucy might become more confident under Mira's influence, and Mira could become more patient with Lucy's sometimes - over - cautious nature.
The fanfiction might explore their relationship in more depth. For example, how they deal with the fact that they are not blood - related but still have a strong bond. It could also show how they support each other in different situations. Like when Lucy is having trouble with her spirits, Mira could be there to give her advice, and when Mira is feeling down about something in the guild, Lucy could cheer her up.