Well, it could be about Harry Potter finding a way to enter Tom Riddle's world in the fanfiction. Maybe he's on a mission to change Tom's path to the dark side.
One possible plot is that Harry is magically transported into a fan - created world where Tom exists. Harry might initially be seen as an outsider but then gradually earns Tom's trust. They could end up learning from each other, with Harry introducing modern magical concepts to Tom and Tom sharing some of the more ancient and perhaps forgotten magic with Harry.
Perhaps Harry joins Tom's eventual lover in order to uncover some secrets that Tom has been hiding. They could start off as distrustful of each other but as they work together, they find common ground. The plot might involve them traveling to different magical places and facing various challenges together while trying to figure out Tom's true nature.
It could be Voldemort. Even if it's a fanfiction where Harry joins Tom, Voldemort's future self might still be a threat and oppose Harry's attempts to change Tom.
Well, one possibility is that Harry discovers a new sense of purpose and discipline in the military. He might face challenges like tough training, and make new friends who help him grow as a person.
Maybe Harry joins the Wizengamot to bring about some long - awaited reforms. He could use his unique experiences to fight against the old - fashioned rules and biases within the magical government.
In this fanfiction, perhaps Harry joins the Fellowship and has to adapt to their way of life. He might share his knowledge of the magical world with the members, like teaching them about potions that could heal wounds faster. And his presence could change the group dynamics, adding a new element of magic to their quests.
Well, it could be that Harry joins the army because he feels a need for a new purpose in life. The story might show him adapting to the strict military discipline. Maybe he'll be part of a special unit and be involved in some sort of mission to save the world in a different way than he did before. For example, instead of using magic directly, he might use his strategic thinking and leadership skills he's developed over time.
In a 'harry joins voldemort fanfiction', perhaps Harry discovers a dark secret about the wizarding world that makes him think Voldemort was right all along. It could be something about the Ministry of Magic being corrupt and Voldemort's way of ruling, although extreme, is the only solution. So he joins Voldemort to 'fix' the wizarding world.
Well, it could be about Harry Potter exploring a new world of magic within the coven. Maybe he discovers new spells or magical abilities that are different from what he learned at Hogwarts. He might also meet new and interesting characters who have their own unique magical practices.
Well, perhaps Harry gets tired of being constantly in danger and feeling unappreciated in the wizarding world. He might be lured by the Dark Side with promises of power and safety. Voldemort could offer him a place by his side, showing him a different view of magic that seems more appealing in a warped way.
Well, perhaps in this fanfiction, Harry discovers his animagus form and the Dark Side somehow convinces him that his unique ability can be better utilized there. Maybe they promise him power and a new understanding of magic that the 'good side' has been suppressing.