Well, leadership is also a key theme. The slime character has to lead his newfound community and make decisions for their well - being. There's also the theme of exploration, as they explore new lands and encounter different races and cultures.
The slime protagonist is definitely one of the most interesting. Its unique ability to absorb and gain powers makes it stand out. Then there's Rimuru, who has a very complex personality and is constantly evolving.
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.
One of the popular ones is probably 'That Time I Got Reincarnated as a Slime'. It has an interesting premise where the main character is reincarnated as a slime in a fantasy world. The story is filled with unique characters, like Rimuru, and explores concepts such as nation - building and different forms of magic. It has a great blend of action, adventure, and humor, which attracts a wide range of readers.