One possible plot is that Harry was held captive by some dark wizards who didn't feed him well. When he's finally rescued, his friends are shocked at his malnourished state. They take him to St. Mungo's Hospital for Magical Maladies and Injuries. The healers there start a special treatment plan for him. As he recovers, he realizes who his true friends are, as they stay by his side throughout his recovery process. And he might start to train again to regain his strength both physically and magically.
It could be Hermione. She's always been very studious and would have read up on all kinds of magical cures for malnutrition. She'd be very determined to help Harry get better.
One common theme could be the struggle of Naruto to overcome the effects of malnutrition while still trying to train and become a great ninja. For example, he might have less energy than his peers, but his determination pushes him forward.
One common situation could be that the other characters in the fanfiction might constantly over - protect Harry. For example, they don't let him take any risks or do things on his own, like going to the Forbidden Forest even for a small exploration. Another situation could be that they talk to him in a very patronizing tone, as if he doesn't understand anything. Maybe they try to feed him or dress him as if he were incapable of doing these basic things himself.
One possible plot could be that Harry's insanity is caused by a cursed object. His friends then have to find a way to break the curse while also dealing with his erratic behavior. For example, Harry might accidentally reveal secrets to the wrong people because of his insanity.
The plot could involve Harry struggling to hide his true nature as a Nathrezim from his friends at Hogwarts. He might have new, dark powers that he has to learn to control, and he's afraid of how others will react if they find out. Meanwhile, Voldemort senses something different about Harry and is both intrigued and threatened by him.
One possibility could be that Harry has been alive for 500 years by using some powerful magic that had unforeseen consequences. He might have seen the rise and fall of many magical civilizations and now has a vast knowledge of magic that no one else does. He could be a sort of hermit, living in an isolated place and only emerging when there is a great threat to the wizarding world that only his ancient knowledge can solve.
In this fanfiction, Harry's Animagus skills as an Avenger might also lead to some interesting character interactions. He could teach the other Avengers about magic and how his Animagus form works. For instance, Tony Stark might be intrigued by the magic behind it and try to find a scientific explanation or a way to replicate it. Meanwhile, Steve Rogers could be more interested in how Harry uses this ability in combat situations.
Well, one possibility is that Harry, now reborn as an Uchiha, might discover his newfound Sharingan abilities early on. He could use his knowledge from his past life to quickly rise through the ranks in the Uchiha clan. Maybe he'd try to prevent the tragedies that befell the Uchiha in the original Naruto story, like the massacre. He could also use his magic - if it somehow transferred with him - in combination with his ninja skills to become a truly formidable force.
One possible plot could be that the contract is a magical binding that forces Harry to work with Voldemort in some way. Maybe it's a deal to save the lives of his friends. So, Harry has to reluctantly follow Voldemort's orders while secretly looking for a way to break the contract.
One possibility could be that Harry is in a difficult situation, perhaps a mystery in a new world that he's not familiar with. He calls Gibbs because he has heard of Gibbs' skills in solving problems and investigations. Gibbs then uses his no - nonsense approach to start looking into the matter, guiding Harry through the steps of finding clues and making sense of the chaos around him.