Perhaps the human, being raised by mermaids, has special abilities. For example, they can hold their breath for an extremely long time and use this to explore deep sea secrets that are unknown to both humans and mermaids, creating a sense of mystery and adventure.
One idea could be the human learning to communicate with the mermaids in their unique language. This could involve lots of misunderstandings at first but then a deep bond forms through shared experiences.
One idea could be the human having unique abilities because of being raised by Pokémon. For example, they might be able to communicate with Pokémon in a special way that normal humans can't. Another could be the human facing challenges in the Pokémon world that are different from those in the human world, like dealing with Pokémon rivalries and trying to keep the peace among different Pokémon species.
One popular plot idea is a mermaid saving a human from drowning and then a romantic relationship developing between them. They might face challenges like the mermaid having to hide her true identity from the human world.
The human's character could develop from being afraid of the underwater world at first to becoming a confident explorer. This change can be shown through their actions, like how they start to swim further and deeper on their own.
One idea could be that Percy has unique powers given to him by Chaos. For example, he might be able to control the very fabric of the universe in small ways. Maybe he can manipulate time on a very limited scale, like slowing it down during a fight to gain an advantage.
One idea could be that Percy is raised by Poseidon. He grows up with a deep understanding of the sea and its powers from an early age. This might lead to him having a more solitary childhood as he has to hide his connection to the god. Another plot could involve Percy being raised by Athena. He would then be extremely intelligent and strategic, perhaps getting into trouble at school for being too smart. A third idea is if he's raised by Apollo. He could have a natural talent for music and healing, and his adventures might revolve around using these gifts in a world that is both dangerous and full of mythical creatures.
Perhaps Percy raised in Atlantis could be involved in a mystery related to the ancient history of Atlantis. There could be a hidden power or artifact that he has to find before the wrong people do. As he searches, he uncovers secrets about his own family that are tied to Atlantis. Along the way, he has to face challenges like underwater monsters that are specific to the Atlantean depths and overcome them using his wits and powers.
One idea could be that Eragon discovers unique dragon magic only accessible to those raised by dragons. For example, he might be able to communicate with all dragons on a deeper level, not just his own. Another plot could involve him facing challenges from dragon - haters who fear his special bond with the dragons. They might try to break the bond or harm him.
Perhaps there could be a plot where Merlin discovers secrets about his past that Nimueh has been hiding from him. This could lead to a rift between them. For example, he might find out that Nimueh had some ulterior motives in raising him, like using him as a weapon against Camelot. And then he has to decide what to do with this new knowledge.
Another plot could involve the 'not - quite - human' character forming an unlikely friendship with a human. Through this relationship, they both learn about acceptance and the differences between their worlds. For example, the human could teach the non - human about things like music or art, while the non - human shows the human a different perspective on life.
One idea could be Gumball and Darwin as humans starting a new school and having to deal with the typical high - school dramas like popularity contests and bullies. Another could be the Wattersons going on a road trip as humans and the various adventures they have along the way. Maybe Gumball as a human gets a part - time job and has to balance work, school and his family life.