A third plotline might be about Harry's friends at Hogwarts becoming aware of his situation. Hermione, with her quick thinking and vast knowledge of magic, could start devising a plan to rescue him. Ron, on the other hand, might be more impulsive and want to rush in to save Harry right away. Their combined efforts, along with the help of some Hogwarts professors who suspect something is wrong, could lead to a dramatic rescue mission.
Another plotline could involve the Dursleys' motives for imprisoning Harry. Maybe they are afraid of his growing magical powers and think that by keeping him locked up, they can prevent him from causing any 'abnormal' incidents. This could lead to a deeper exploration of the Dursleys' insecurities regarding magic. Also, Harry could befriend some magical creatures that somehow find their way to him in his imprisoned state, like a house - elf who decides to help him escape or at least make his situation more bearable.
One possible plotline could be Harry finding a way to secretly practice magic while imprisoned. For example, he might discover a hidden corner in the attic where he can perform small spells without the Dursleys noticing.
Hedwig could be a potential ally. She could carry messages to Harry's friends at Hogwarts, alerting them of his situation. Even though she is just an owl, she has shown loyalty and intelligence in the original series.
Perhaps in the fanfic, a time - traveling wizard goes back to save Harry from the Dursleys. This would lead to a lot of changes in the timeline. Harry might be introduced to magic much earlier and form different relationships. He could also have a different understanding of his parents' history and his own role in the wizarding world as a result of being removed from the Dursleys' environment.
One possible plotline could be how the Dursleys face magical mishaps in their daily lives that drive them crazy. For example, maybe Dudley accidentally gets turned into a pig by a stray spell and the Dursleys have to deal with that chaos. Another could be that they are constantly haunted by strange magical creatures that are attracted to Harry's presence in their house.
One possible plot development could be that Harry discovers his magic earlier on his own due to the neglect. He might start exploring the wizarding world secretly, finding hidden magical artifacts in the neighborhood. Another could be that he forms an unexpected alliance with the local magical creatures that sense his isolation. Maybe he even starts teaching himself basic spells from an old, dusty spellbook he finds in the attic.
A plotline might involve Harry being reincarnated as Smaug in a parallel universe. As Smaug, he remembers his past life as Harry and tries to use his knowledge of magic to rule over the lands in a more benevolent way than the original Smaug. Or perhaps there is a prophecy that states only a wizard - turned - dragon can save both the wizarding world and Middle - earth from a great evil, and Harry is the chosen one to become Smaug.
One possible plotline could be that Harry Potter sacrifices himself to defeat an even more powerful dark wizard than Voldemort. Maybe this new enemy has a power that can only be countered by Harry's self - sacrifice, which would lead to a very bittersweet ending for the wizarding world.
In some fanfics, when Harry leaves the Dursleys, he might immediately feel a sense of freedom. He could start exploring the wizarding world more independently, without the constraints of his Muggle relatives. Maybe he'd meet new friends or allies earlier than in the original story.
In certain fanfictions of this type, Harry could be found by a powerful but reclusive wizard. This wizard decides to mentor Harry in the ways of ancient and lost magics. Harry then uses this knowledge to not only become a very powerful wizard but also to change the wizarding world's view on things like blood purity. He may also go on a journey to find out why the Dursleys abandoned him and in the process, uncover some dark secrets about his family that had been long buried.
One possible plotline could be Naruto trying to find a way to visit the Harry Potter world. He might search for some kind of magical portal or a powerful spell that can transport him there. Another plotline could be Naruto using the ideas from Harry Potter books to improve his ninja techniques. For example, he could learn about the concept of wandless magic and try to apply it to his chakra control.
One possible plotline could be that Harry is cursed and slowly turns into a dragon. As he changes, he has to deal with the physical and mental challenges of this transformation. His friends would try to find a way to reverse the curse while Harry struggles to control his new instincts.