One idea could be that Harry travels back in time and decides to adopt his younger self to protect him from all the hardships he faced. Maybe he uses magic in ways that haven't been explored before to create a new identity for himself as his own 'adoptive parent'.
In the 'Harry Adopts Himself' fanfiction, it could be that Harry, after defeating Voldemort, realizes that he could have saved himself a lot of pain if he had been guided better. So, he uses some powerful time - magic to go back and adopt himself. He then takes his younger self away from the Dursleys and starts training him in a secret location, away from the prying eyes of the wizarding world.
Perhaps in this fanfiction, Harry discovers a spell that allows him to split his consciousness. One part of him then takes on the role of the adoptive father. He could use his knowledge of the future to teach his younger self things that Dumbledore or others didn't teach him, like more advanced defensive magic right from a young age.
In 'Loki adopts Harry Potter' fanfiction, it could be that Loki adopts Harry due to some sort of prophecy. Loki realizes that Harry is crucial in a much larger plan involving both the wizarding world and the Nine Realms. So, he takes Harry under his wing. He might train Harry not only in magic but also in strategy and deception. Harry could then use these skills to form alliances within the wizarding world that were previously unthinkable, like getting the Slytherins to side with him against Voldemort by using Loki - inspired tactics.
One plot idea could be that Harry gives himself to Voldemort in an attempt to understand the Dark Lord's mind and find a weakness from within. Another could be that he sacrifices himself this way to protect his friends and Hogwarts, believing that there's a way to turn Voldemort's victory into a defeat from the inside. Or perhaps Harry has a vision that shows this as the only way to truly end the war and he decides to take the risk.
A common plot might involve Grindelwald taking Harry away from the Dursleys much earlier than in the original story. He could introduce Harry to a different side of the wizarding world, filled with ancient and dangerous magic. Harry would then have to decide whether to embrace Grindelwald's vision or fight against it, all while dealing with his own identity and the power he possesses.
Maybe a Pikachu adopts a human child. The Pikachu is part of a group of Pokemon that live near a human village. The child was abandoned, and the Pikachu takes it in. They go on adventures together, with the Pikachu protecting the child from other wild Pokemon and teaching it how to communicate with different Pokemon species.
Perhaps a key plot point is the reaction of his old friends like Ron and Hermione. They might be shocked and try to convince him to reverse his decision. They could struggle to understand his reasons, which could lead to some complex character dynamics.
In 'Dumbledore adopts baby Harry fanfiction', an interesting aspect could be how Dumbledore chooses to educate Harry. He might introduce him to different magical cultures and theories much earlier than in the original story. Also, the relationship between Dumbledore and the other Hogwarts staff when they find out he has adopted Harry. Some might be supportive, while others might worry about the potential implications for Hogwarts and the wizarding world as a whole.
Well, in such fanfiction, a great plot point could be the relationship between Snape and Harry evolving from animosity to a father - son like bond. Also, Harry might learn some of Snape's more secretive Potions techniques, which could be used to his advantage in the wizarding world. He could also face challenges from other Slytherins who are not happy about Snape's special treatment of Harry.
One idea could be that Percy adopts a young demigod who has a very rare and powerful ability. Percy has to teach this new demigod how to control their powers while also protecting them from monsters who are after the new kid because of their potential. Another could be that the adopted demigod is from a different pantheon entirely, which causes some interesting cultural clashes and misunderstandings within the Camp Half - Blood environment.
In some of these fanfictions, an interesting plot development could be Severus having to deal with the boys' different personalities. Harry, being brave and impulsive, while Draco is more reserved and proud. Severus has to find a way to balance their needs. For example, when it comes to their magical education, he might have to teach them in different ways. He could also have to handle the outside world's reaction to him adopting both a former 'enemy' (Harry) and a pure - blood Slytherin (Draco). Another aspect could be how the boys form a bond with each other through Severus' influence. They might start to see past their initial differences and become like brothers, which changes the whole dynamic of the wizarding world they live in.
Hermione could be a big part of it. She might be the one who discovers the magic or the theory behind Harry being able to adopt himself and helps him work out the details. She could also help with taking care of the younger Harry, teaching him things like how to study magic more effectively.