One interesting plotline could be Percy adjusting to the Underworld. He might have to learn new rules and customs, like how to interact with the various spirits and creatures there. Another could be Hades trying to train Percy in unique ways, perhaps teaching him powers related to the Underworld that he didn't have before.
A plotline might involve Hades having a prophecy that Percy is crucial to his future plans. This makes him possessive as he doesn't want Percy to be influenced by other gods. He could start by subtly manipulating events around Percy. For example, causing problems in Percy's relationships so that Percy feels more alone and might turn to Hades for support. Hades might also be possessive because Percy has a unique connection to the Underworld, like being able to travel there easily, and Hades wants to keep that power close to him.
Well, a frequent plotline is that of Fem Percy being lost in the Underworld and Hades having to deal with her presence. This could lead to a series of events where Hades is initially annoyed but gradually comes to respect Fem Percy's abilities. Another common one is where Hades offers Fem Percy a deal. It could be for her soul or for some other valuable thing in exchange for her help in a matter that only she can solve. And then there are plotlines where Fem Percy is on a quest that somehow intersects with Hades' domain, and they have to navigate their relationship while she tries to complete her mission.
Obviously, Percy and Hades will interact a great deal. Percy will also likely interact with the Furies, as they are servants of Hades. They could be like his protectors or trainers in some sense. Another character could be Nico di Angelo, since he has a connection to Hades as well. They could bond over their shared relation to Hades.
Another plotline might involve Itachi protecting Naruto from the Akatsuki. Since Itachi has his own motives within the Akatsuki, he could use his position to keep Naruto safe from the other members who might be after the Nine - Tails. It would add a lot of tension and drama to the story, especially when Itachi has to balance his loyalty to the Akatsuki and his new - found role as Naruto's protector.
One interesting plotline could be how Dumbledore helps Harry discover his true magical potential beyond what he learned at Hogwarts. For example, he might introduce Harry to ancient and rare magic that was long forgotten. Another could be Harry's relationship with the wizarding community changing as he is now seen as Dumbledore's adopted son. This could lead to new friendships and alliances. Also, Dumbledore might train Harry to become a great leader in the wizarding world, preparing him for future threats that are much larger than Voldemort.
One possible plotline could be Hermione using her intelligence to create a stable and loving home environment for Harry. Maybe she helps him deal with his PTSD from his past traumas. Another could be them facing magical threats together as a family, with Hermione teaching Harry new spells and ways to protect himself.
In some 'Snape Adopts Harry' fanfictions, there is a plotline where Snape and Harry have to work together to uncover a mystery at Hogwarts. This mystery could be related to the Dark Arts or some hidden secrets of the school. Along the way, their relationship evolves from a reluctant guardian - ward relationship to a more loving father - son bond. Harry might start to understand Snape's past actions and Snape, in turn, learns to open up and be more affectionate towards Harry.
Well, sometimes in these fanfictions, a great plotline is Snape trying to protect Luna from the bullies at Hogwarts. Given Snape's own experiences of being bullied as a child, he would be very determined to make sure Luna doesn't go through the same. He might use his position as a teacher to subtly intimidate the students who pick on her. Also, there could be a plot where Luna starts to teach Snape about the beauty of the natural magic around them. Snape is so focused on the academic and often darker aspects of magic, that Luna's view of the magic in simple things like a butterfly's flight or the way a flower blooms could be eye - opening for him.
Well, in some of these fanfictions, the plot could involve Harry having to adapt to a completely different world from the wizarding one. Gibbs would be like a father figure, guiding him through the Muggle world's rules and values. There could also be a plot where Harry's past with Voldemort makes him a target in the Muggle world, and Gibbs has to protect him while also trying to understand this whole new concept of magic.
Well, in some 'Draco Adopts Harry' fanfictions, a great plotline is how Draco overcomes his past enmity towards Harry. He starts to see Harry as a person in need rather than an enemy. There could be a plot where they work together to uncover secrets about Harry's past that Dumbledore might have hidden. And then, Draco has to protect Harry from those who don't want the truth to come out.