A single adventurer could be the main character. This person might be a thrill - seeker who built a shrinking device and decided to explore the human body. Along the way, they make friends with some helpful bacteria and together they face the challenges of the body's inner environment.
One idea could be about a microscopic adventure where tiny characters explore the human body like it's an uncharted world. They could start in the bloodstream and ride the red blood cells like boats, facing challenges such as avoiding white blood cells that might mistake them for invaders.
The rainbow of the human body was a mysterious creature that possessed a seven-colored spectrum that could produce unique color changes within the body. According to legend, this creature lived in ancient times and once caused great harm to humans. Later, it was destroyed by a mysterious mage. However, the rainbow of the human body did not exist in reality. It only existed in novels.
A possible plot is that the control panel in Riley's mind gets a malfunction, and the emotions have to work together in a completely new way. They might have to swap their usual functions, so Sadness has to take on the role of Joy for a while. This could lead to some really strange behaviors from Riley on the outside. Also, what if an outside force, like a dream character, tries to invade the inside world of Riley's emotions and disrupt their harmony?
The One Thought Eternal World wasn't within the human body. One Thought of Eternity was a Xianxia novel. It was about a timid and cowardly person who gradually grew up and eventually became a mighty figure. The novel's worldview did not mention that it was in the human body. Therefore, the One Thought Eternal World was not inside the human body.
Here are a few recommended novels about human body modification:
1. Song Rongyu was a gentleman from a wealthy family, but he fell into a deep sleep because he refused to inherit the family business. Zhao Xinyue was a dream rescuer. Her mission was to attack Song Rongyu in the dream, hurt him, and play with his feelings. This novel depicted the intersection between reality and dreams, as well as the story of human modification.
2. "The Ragged Cow": This novel described the female protagonist's experiences with a series of human modifications, including being tied up, being whipped with a rolling pin, and being forced to do R. The story was full of ups and downs, full of tension and excitement.
3. "My Countless Doppelgangers": Although it wasn't a lobster-style human body modification, this novel described the protagonist's superhuman clone ability, which satisfied the needs of the human body modification.
4. "Life with a Different Brain": The protagonist's brain was expanded through a pill and transformed by body simulation software, giving him various abilities. This novel showed the wonderful journey of the abnormal brain's life.
I hope that the novels recommended above can satisfy your needs for human body modification.
To create believable characters in this fanfiction, start with the existing emotions from the movie. Study their personalities and how they interact. For example, Joy is always positive and energetic, so keep that in mind when writing her. Then, add depth by giving them new experiences. If Anger has to deal with a situation where he can't be angry, how would he react? This kind of development makes the characters more believable.