For Dimentio, show his internal conflict. He might be torn between his loyalty to a certain group and his newfound friendship with Mimi. As for Mimi, make her grow. Let her be more than just a one - dimensional character. For example, she could start as a bit of a spoiled character but through her experiences with Dimentio, become more selfless and brave.
Well, it could be a love - story - ish kind of fanfic. Dimentio and Mimi might start off as just acquaintances but gradually develop feelings as they are forced to work together on some important task in their world. They could have cute interactions and moments of jealousy from other characters.
Obviously, Naruto and Dimentio are the main characters. Naruto, the energetic ninja from the Hidden Leaf Village, and Dimentio, the rather mysterious character from another realm.
Well, it could be about Naruto somehow being transported to the world where Dimentio exists. Maybe Naruto's ninja skills would be a novelty there and he'd have to team up with Dimentio to face some common enemy. They'd learn from each other's abilities, like Naruto teaching Dimentio about chakra and Dimentio showing Naruto some unique magic tricks.
One common theme is friendship. Since both characters have their own complex personalities, exploring how they form a bond can be interesting. For example, Mr. L's struggle with his identity and Dimentio's somewhat mysterious nature could lead to a unique friendship development in the fanfic.
For Arya, focus on her growth from a vengeful girl to a more complex character. Show her learning from Jaqen, not just in terms of skills but also in understanding different ways of life. Let her struggle with her identity as she is influenced by Jaqen.
To develop Ahsoka's character in the fanfic, focus on her Jedi training and how it influences her actions. For instance, her sense of justice and her ability to stay calm in dangerous situations. You can also show her growth from her experiences in the Clone Wars. For Lux, emphasize his idealism and his desire to make a difference. Maybe he has some skills from Onderon that he brings to his adventures with Ahsoka.
To develop Brittany, you can focus on her extroverted side. Give her a passion for something like dancing. Describe how she lights up when she hears her favorite song and can't resist moving to the beat. For Jeanette, emphasize her intelligence. Maybe she has a photographic memory when it comes to books she's read. Show how she uses this knowledge in different situations, like solving a mystery in their town.
Another place could be FanFiction.net. However, the quality can vary a great deal. Some stories might be really well - written, while others may not be as good. But it's still worth checking out. Look for the specific tag 'Dimentio x Mr L' to narrow down your search.
I think Mario is more popular. It's a classic and widely known franchise with a huge fan base.
I don't have enough information to say exactly who Mimi is in this story. She could be a character from a game that the Game Grumps were playing.