One popular tensura fanfic might be those that explore different character relationships. For example, stories that focus on Rimuru's interactions with his newly formed kingdom's citizens in more in - depth ways. It could show how his leadership impacts individual lives.
Start by creating an outline. Decide on the main plot, the sub - plots, and how the characters will develop throughout the story. If you're going to introduce new characters, make sure they fit well into the Tensura world. And don't be afraid to add your own unique twists to the story. For example, you could make Rimuru face a challenge that is completely different from what he has faced in the original series, like a moral dilemma that tests his leadership in a new way.
The plot involves Rimuru's journey from being a rather weak (initially) slime to becoming a very powerful being. He meets many interesting characters. For example, there are the goblins he befriends early on. Rimuru helps them improve their living conditions. As the story progresses, he also has to deal with the threat of other more powerful monsters and even some human kingdoms that view his growing power with suspicion. In general, it's a story full of adventure, growth, and the forging of relationships in a magical and dangerous world.
The main character is Rimuru Tempest. He is a slime who has unique powers and a great deal of charisma. Another important character is Veldora, the storm dragon who befriends Rimuru. There is also Shizu, a human who has an impact on Rimuru's early development. And then there are characters like Benimaru, a powerful ogre who becomes one of Rimuru's loyal followers.