One idea could be a story where Arthur and Mera go on an adventure to a hidden underwater kingdom that has been forgotten by most. They could face challenges like ancient sea creatures and mysterious magic.
Perhaps in the fanfiction, the Mera Mera no Mi's power is modified when Naruto has it. Since Naruto has a large amount of chakra, the fire produced by the fruit could be enhanced in a way that it can even burn through powerful ninjutsu defenses. For example, when facing Sasuke's Amaterasu, Naruto's Mera Mera no Mi - enhanced fire could counter it in a unique way.
The writing style also matters. If the author can create a vivid and immersive world, it will draw the readers in. Using descriptive language to portray the underwater kingdom, the powers of Arthur and Mera, and their emotions towards each other will make for an excellent fanfic.
One great 'Aquaman Arthur and Mera Fanfic' is 'The Depths of Their Love'. It beautifully explores the relationship between Arthur and Mera. The author does an amazing job of delving into their characters, showing how they support each other through the various challenges in Atlantis. It's filled with vivid descriptions of the underwater world and their adventures together.
Since we have no details about the story, it could be a legend passed down in Atlantis. Maybe it's a story about a great hero from their past, and Mera is telling Arthur this story to inspire him or to give him some historical context that could be relevant to their current situation. It could also be a story about the magic or unique powers that exist in their world, which Arthur may need to understand better for some future task or challenge.
One idea could be exploring their relationship before Arthur became king. Maybe they met as young people and had a secret friendship that gradually turned into love.
On Archive of Our Own (AO3), there are likely some great Arthur Mera fanfic authors. You can search for their stories and then check out the other works by the writers who create engaging stories about them.
There could be a fanfiction where the Mera Mera Fruit is a key element in a crossover between the Naruto and One Piece worlds. Naruto travels to the One Piece world and obtains the fruit there. As he tries to master it, he meets various One Piece characters who help him understand the true potential of the fruit. This leads to a series of adventures where Naruto uses his ninja skills along with the fruit's powers to overcome challenges and make new friends in this strange new world.
One idea could be an Arthur crossover with the world of Alice in Wonderland. Arthur could find himself in Wonderland, meeting the strange characters and getting into all sorts of wacky adventures. Another could be a crossover with a superhero universe, where Arthur gains superpowers and has to save his world from a new threat. A third idea might be an Arthur crossover with a historical setting, like the Middle Ages, where he has to adapt to the different time period and its challenges.
She might tell Arthur the story to make him understand something better. For example, if it's a story about Atlantis' history, she could be trying to make him more aware of his heritage and the significance of his role in their world. Or it could be a story of a past mistake that she doesn't want Arthur to repeat.
One idea could be that Arthur discovers a new hidden tribe of invisibles. They have a unique power that can help save the Invisibles' world from a new threat. Another could be Arthur goes on a quest to find a magical object that was lost long ago in the Invisibles' realm, which is said to grant the finder a special wish.