Another plot could involve a villain from the Fairy Tail universe following Lucy into the One Piece world. This villain might be after a powerful Devil Fruit or some ancient treasure in the One Piece world. Lucy then has to rely on the Straw Hat crew's help to defeat the villain while also trying to find a way back to her own world.
One interesting plot could be that there is a prophecy about Lucy's Star Dress. It says that it will be the key to saving the entire magical world from an ancient evil. But there are also those who want to use the Star Dress for their own evil purposes. Lucy has to protect the Star Dress and fulfill the prophecy, all while dealing with the various factions that are after it.
One idea could be that Naruto and Lucy end up in a parallel universe where they have to combine their powers to find a way back home. Naruto's ninja skills and Lucy's celestial magic could be used in creative ways to overcome the obstacles in this strange new world.
She could fit into the power structure by forming alliances. Since the One Piece world is full of various factions, Lucy could side with the Straw Hat Pirates. Her powers would be useful in their adventures. She might also encounter other groups like the Revolutionary Army. Her celestial spirit magic could be used for espionage or gathering information for them. And as she travels more in the One Piece world, she could learn about new ways to enhance her magic using the unique resources there.
She could use her celestial keys to open portals to different islands in the One Piece world. This would be a really cool way to travel and could also lead to some interesting encounters. For instance, she might end up on an island full of devil fruit users and have to use her magic to survive.
In a fanfiction, Lucy might be a new addition to the Grand Line. She could bring a different kind of power dynamic. Her magic could be seen as a rare and valuable asset. The Straw Hats might initially be skeptical of her, but as they see her power in action, they welcome her into the crew. And she could learn from the different fighting styles in One Piece, like Luffy's Gum - Gum attacks.
Another plot could be that Lucy gets pregnant through some sort of magical accident. She then has to figure out how to deal with the changes in her body and magic while also trying to find a way to reverse the 'accident' if she wants to. But during this process, she starts to form a special connection with the unborn child and has a change of heart.
Well, a plot could revolve around Lucy being the only one who can stop an ancient Celestial Dragon that has been awakened from its slumber and is causing chaos. As a Celestial Dragon Slayer, she has to figure out how to communicate with the dragon first and then find a way to either calm it or defeat it. This could lead to her delving deeper into the lore of Celestial Dragons and her own family's connection to them.
One idea could be that Whitebeard is transported to a different time in the One Piece world. Maybe he ends up in the future and has to deal with new pirates and powers that he's never seen before. Another could be a crossover with another anime world, like he somehow gets stuck in the Naruto world and has to find his way back while dealing with ninjas.
Perhaps a villain from DBZ, like Frieza, finds a way to travel to the One Piece universe. He starts enslaving the pirates and taking over islands. Luffy and his crew would be the first to resist, but they quickly realize they need help. Goku and Vegeta then arrive and have to teach the pirates about ki and how to fight against Frieza's powerful minions. It would be a great opportunity to see the different fighting styles blend together.
One idea could be Mihawk getting transported to a different part of the One Piece world where he has to team up with the Straw Hats to face a new and powerful enemy. Maybe a group of ancient sea gods that are awakened and threatening to destroy all pirates. The Straw Hats could initially be wary of Mihawk but soon realize his skills are crucial for their survival.