One idea could be a crossover where Naruto and Anna Heartfilia team up to fight a common enemy from both of their worlds. Maybe a powerful dark wizard from Anna's world is trying to steal some of the ninja techniques from Naruto's world. Another plot could be that Naruto somehow ends up in Anna's world and has to learn how to adapt to the magic there while also helping Anna with a personal quest she has. For example, she might be trying to find a long - lost magical artifact and Naruto's skills come in handy.
A really interesting plot could be a time - travel scenario. Anna somehow travels back in time to Naruto's childhood. She becomes a mentor - like figure to him, guiding him in ways that Jiraiya or Iruka couldn't. This changes Naruto's development and future, and they end up changing the entire Naruto universe for the better.
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.
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.
A plot could involve a shamanic tournament in the Naruto world, similar to the one in Shaman King. Naruto, Sasuke, and other ninja characters participate, and Anna is brought in as a special guest or judge. Along the way, Naruto discovers new powers within himself that are related to shamanic energy, and he has to learn how to control them with Anna's help.
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 possible plot could be that Naruto is somehow transported to the Fairy Tail world where he meets Lucy Heartfilia. They could team up to fight a common enemy that threatens both their worlds. Naruto's unique ninja skills combined with Lucy's celestial spirit magic would make for some epic battles.
Naruto is a very passionate person. He wears his heart on his sleeve and is not afraid to show his emotions. Anna Heartfilia, while also passionate in her own way, is more guarded. In a fanfiction, Naruto's open - heartedness could gradually break down Anna's walls. He might share his own stories of loss and how he overcame them, which would resonate with Anna. As they interact more, Anna would start to trust Naruto and share her own secrets and fears. Their different personalities would create a rich dynamic, with Naruto bringing out the more carefree side of Anna and Anna teaching Naruto about the subtleties of magic and the importance of family heritage.
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.
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.
A plot could revolve around their concerns and hopes for the future. Since they are royalty, they might worry about how their child will be raised with the responsibilities of the throne. They could have conversations with Elsa about it, seeking her advice. Also, it would be interesting to see how the townspeople react to the news of the pregnancy. Some might be overjoyed and start preparing gifts and celebrations in advance.