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.
In this fanfiction scenario, Dark Fem Harry joining Voldemort might lead to a re - evaluation of the prophecies. Since she's changed sides, the prophecies that were thought to be about her saving the wizarding world might now be seen in a different light. She could also start to recruit other disillusioned wizards to Voldemort's cause. This would expand his army and make the fight against them even more difficult. Additionally, her new allegiance could cause a rift in her previous friendships, like with Hermione and Ron, who would now be forced to see her as an enemy and try to stop her by any means necessary.
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.
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 interesting plot twist could be that Fem Harry's presence among Voldemort's ranks causes internal conflict. Some Death Eaters might be distrustful of her because she was once the 'Boy Who Lived' in a female form. Another twist could be that Fem Harry starts to influence Voldemort's ideology in a way he didn't expect. Maybe she makes him question his pure - blood supremacy by showing that her Muggle - born friends were as powerful as any pure - blood wizard. Also, it could be that her joining was part of a long - con plan to bring down Voldemort from the inside, and she gradually starts to turn his own followers against him.
Her character would likely become more ruthless. Since she's joining Voldemort, she'd have to adapt to his ways of using dark magic without hesitation. For example, she might no longer have the moral qualms about using the Unforgivable Curses.
In the 'Harry gets captured by Voldemort fanfiction', Harry could find an unexpected ally within Voldemort's ranks. Maybe a Death Eater who has a change of heart. This ally could help Harry escape or at least send messages to his friends. Another aspect could be that Voldemort performs some dark magic on Harry that gives him strange powers or visions. Harry then has to figure out how to use or overcome these new abilities while still being held captive. It could also be that the wizarding world outside is in chaos due to Harry's capture and different factions start to emerge, some wanting to negotiate with Voldemort for Harry's release and others wanting to go to war immediately.
Maybe the story could focus on Harry trying to find a way to reverse the situation. He could go on a quest to find some ancient magic or a powerful wizard who might be able to help. There could also be a plot where the Ministry of Magic gets involved, either trying to cover it up or use it as a weapon against Voldemort. And of course, there would be the internal struggle for Harry, dealing with his own feelings about Voldemort and this unexpected pregnancy.
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, one possible plot development could be that Harry uses the cane as a symbol of his new dark power. Maybe he starts to use dark magic in ways that are unexpected, like using the cane to channel dark energy for attacks. He could also start to turn against his old friends, causing a rift in the wizarding world.
Well, one possibility is that Harry is tricked into joining the dark side. Maybe Voldemort uses some dark magic to cloud Harry's mind and make him believe that the dark ways are the right ones. He could start using dark spells and turn against his friends.
It could be that the Dark Lord offers Harry something irresistible. Like a way to bring back his parents. Harry, being desperate to have his family back, might be tempted to join the Dark Lord in the fanfiction. After all, the love for his parents was always a strong driving force in his character, and this could be a major turning point that leads him to side with the Dark side.