The unique concept of being reincarnated as a slime is really interesting. It allows for a fresh take on the isekai genre. Also, the way the slime, Rimuru, builds his own community and kingdom is engaging.
One interesting plot could be Naruto having to adjust to his new slime form. For example, he might struggle with not having hands in the traditional sense at first but then discovers unique ways of using his slime body for combat or daily tasks. Another plot could be Naruto using his ninja knowledge in the slime world. He could teach the slimes some of his ninja techniques, like how to move stealthily or use basic chakra concepts in their own slime - based magic system.
One common theme is power exploration. Since Rimuru has unique powers in the original series, many fanfictions explore different aspects of his powers or even give him new powers. Another theme is relationship building. Rimuru's relationships with his various friends and allies are often expanded upon in fanfiction.
Rimuru is a must - include character. As the main character in the original work, he can be the central figure in fanfiction too, with new adventures or relationships. Another popular one is Veldora. His powerful and somewhat comical personality can add a lot of fun elements to the fanfiction.
One idea could be the main character exploring a new, hidden part of the slime world that has unique magic and creatures. For example, a place filled with sentient plants that can communicate with slimes.
Rimuru starts as a rather weak slime, but as the story progresses, he evolves through various means. He absorbs knowledge from the beings he assimilates, which broadens his understanding of the world. His experiences with forming alliances, resolving conflicts, and building his own territory contribute to his growth. He becomes more confident, more strategic, and more empathetic towards others, which are all signs of his remarkable growth throughout the novel.
It depends on your taste. If you like isekai (reincarnation in another world) stories, especially those with a non - human protagonist like a slime, then it's probably worth reading. It offers a fresh take on the genre with its own set of rules and concepts for the slime's abilities and growth.