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.
A historian could also make an interesting main character. They could have in - depth knowledge of different forms of governance and use that to help build better kingdoms in the Tensura world. They might also know about ancient battles and strategies that could be applied in the new world.
Perhaps the reincarnated character has a special ability that allows them to communicate with the unique monsters in the Tensura world. This ability could lead to forming alliances that no one else could, and they could use these to their advantage to gain power and protect the world from new threats.
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.