Perhaps she has already had some experiences in the wizarding world before coming to Hogwarts due to her older age. This could lead to a plot where she is being hunted by dark forces even before she starts at Hogwarts. And her time at Hogwarts could be more about using the school as a base to fight back against those threats while also trying to keep her friends safe.
In such a fanfiction, perhaps the plot could involve fem Harry using her wits to outsmart Voldemort while in the guise of a slave. She might be able to gather information about his plans and pass it on to the Order of the Phoenix. But it's a very dark and unorthodox take on the story. Usually, the Harry Potter story is about good versus evil in a more straightforward way, and this would be a major deviation. There could also be a plotline where Harry tries to find a way to break the slave bond through some long - forgotten magic or ancient spell, which would add an element of mystery and adventure to this rather dark premise.
Well, it could be that her joining Voldemort causes a huge shift in the balance of power. The wizarding community would be in chaos as they try to figure out how to deal with this new threat. Dark Fem Harry might also start to influence Voldemort in unexpected ways. Maybe she convinces him to use more psychological warfare rather than just brute force. And she could be the one to find and destroy some of the remaining Horcruxes of other wizards, believing that it would make Voldemort even more powerful and thus her position more secure.
One possible plot development could be that Harry tries to find a way to escape the marriage. She might use her magical skills and the help of her friends to break free from Voldemort's control. Another could be that she pretends to go along with the marriage while secretly planning to undermine Voldemort from within.
One possible plot development is that Naruto could be involved in the Uchiha clan affairs much earlier. Since he's older, he might have the power and influence to prevent the massacre. He could use his charisma and strength to broker peace between the Uchiha and the village leadership. Another development could be that Itachi becomes Naruto's apprentice of sorts. Naruto could teach Itachi some of his unique jutsu or ways of thinking that are different from the traditional Uchiha teachings.
Well, if fem Harry is older, it might change her relationships a great deal. For example, in her friendship with Ron and Hermione. She could be more of a mentor figure to them rather than just a peer. Older fem Harry might have a more mature outlook, which could lead to some initial friction as her friends might not be used to this new dynamic.
One common theme is the exploration of character growth over a longer period. Since the characters are older, the authors can show how they have developed from their younger selves in the canon. For instance, Harry might be more world - weary and wiser.
Perhaps in the 'harry is the heir of pendragon fanfiction', Harry's new status as the heir of Pendragon causes a stir in the wizarding world. Some might fear his new - found power, while others might see him as a great hope. He could be drawn into a political struggle within the magical community as a result.
Well, one possible plot development could be that Harry learns some really unique combat skills from Scathach. Maybe he becomes a much more formidable wizard in dueling. Another could be that he gains a different perspective on magic through her teachings, which helps him in his battles against Voldemort.
One possible plot development could be that Voldemort fully regains his power without Harry as an obstacle. The wizarding world would be plunged into complete darkness. His friends, like Hermione and Ron, might go into a deep state of despair and then form a resistance group. They would try to find other ways to defeat Voldemort, perhaps by seeking out more powerful magical artifacts or uniting with some previously neutral magical creatures.
One possible plot development could be how the wizarding world reacts to the loss of the so - called 'Chosen One' even as a baby. There might be widespread grief and fear among those who opposed Voldemort, as they had placed so many hopes on Harry. Another aspect could be how Voldemort's reign of terror continues without the threat of Harry Potter. He might become even more powerful and tyrannical, with no one to oppose him in the way Harry was prophesied to.