Power growth is a very common theme. Since the character is starting as a monster, often a weak one, they have to figure out how to become stronger. For example, in many of these fanfictions, the reincarnated monster will hunt for food or resources that can enhance their abilities.
Another common theme is adaptation. The main character has to adapt to their new form. If they are a slime, they have to learn how to move, communicate, and interact with the world in a way that suits their gooey, shapeless body. This often leads to some creative and humorous situations as they struggle to do things that were easy in their previous human form.
The exploration of a new world is another typical theme. When reincarnated as a monster, the character is in a different world or at least a different part of their original world. They get to discover new magic systems, cultures of other monsters, and hidden secrets. For instance, in many of these novels, the protagonist discovers unique magic spells or abilities that are specific to the monster species they have been reincarnated into.
Survival is a common theme. When the protagonist is reincarnated as a monster, they often have to fight for their lives in a world that may be hostile. For example, in 'So I'm a Spider, So What?', the spider has to constantly find food and avoid being killed. Another theme is growth. They usually start weak and gradually become stronger through various means like leveling up or evolving.
One common theme is redemption. Firestar may have made mistakes in his past life and in his reincarnation, he tries to make up for them. For example, if he had failed to save a certain cat in his previous life, he might strive to protect that cat or its descendants in his new life.
One common theme is identity. Percy has to figure out who he is in his new life. Is he still the same hero? Another theme is redemption. Maybe he made some mistakes in his past life and his reincarnation gives him a chance to make things right.
One common theme is self - discovery. Since Hermione is reincarnated, she often has to figure out who she is in this new life, especially when there are changes in her family or social status.
One common theme is redemption. If in her previous life she made mistakes, in her reincarnated form she tries to make up for them. For example, if she hurt someone in the past, she might go out of her way to help that person in the new life.
One common theme is redemption. Since Snape had a complex and often misjudged life in the original series, many fanfictions focus on him getting a second chance to redeem himself in his new life. For example, he might correct the mistakes he made with Lily or his students.
A frequent theme is the use of prior gaming knowledge. The protagonist will use their understanding of game mechanics such as stats, leveling up, and strategy to their advantage in their new animal life. For instance, if reincarnated as a wolf, they might use strategies like those in RPGs to hunt more effectively. Another theme can be the exploration of a new world. As an animal, they explore areas that were inaccessible before and uncover secrets of the animal kingdom.
Another theme is changing the story. Reincarnated characters often try to change the events that happened in the original Dragon Ball plot. They might prevent certain deaths or form different alliances. For instance, they could try to stop Vegeta from being evil in his early days in the series.
One common theme is self - improvement. The OC, with the help of the system, usually tries to become stronger, whether it's in terms of physical abilities, magical powers or knowledge. Another theme is exploration. Since they are in a new world, they explore different places, cultures and meet new races. For example, in many such fanfictions, the OC might explore a hidden dungeon or a lost city.