Another aspect could be his internal struggle. Harry might have nightmares and visions related to the Dark Mark. He'd be constantly on edge, worried about being controlled by Voldemort. His relationships with his friends, like Ron and Hermione, would be severely tested. Hermione might try to research ways to remove the Dark Mark while Ron would be more impulsive, wanting to directly confront the situation, which could lead to conflicts among them.
In some versions, Harry might struggle with the internal conflict of having the Dark Mark. He could face discrimination from those who believe he has truly turned to the dark side. This could lead to a situation where he has to prove his innocence while also dealing with the newfound power or connection that the Dark Mark might bring.
In this fanfiction, it could also be that Harry's Gryffindor nature starts to rub off on Snape in some ways. They might have arguments about how things should be done, but through these interactions, Snape might start to see the value in Harry's way of thinking. And Harry, on the other hand, might learn some of Snape's more cautious and calculating ways, which could help him in his future battles.
Obviously, Voldemort would be a main antagonist. Since it's the Dark Mark of Voldemort that Harry is forced to take, Voldemort would be pulling the strings, trying to make Harry do his bidding.
Perhaps there could be a magical object or spell involved that is causing this forced marriage. Harry and Snape might have to work together to find and break this spell. Along the way, they could learn more about each other's pasts and secrets, which could change their relationship from one of enmity to something more complex.
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.
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 Dark Harry has some ulterior motive for kidnapping Hermione. Maybe he wants to use her knowledge to further his dark plans. For example, Hermione is known for her intelligence and vast knowledge of magic, and Dark Harry might force her to help him find a powerful and dark magical artifact.
Well, one possible plot development could be Harry struggling with his new identity. He might face inner turmoil as he tries to come to terms with being the son of the Dark Lord. Another could be the reactions of his friends. Some might turn against him, while others could try to help him find a way to break free from the dark legacy. And there could also be a plot where the wizarding world is divided on how to deal with Harry, with some seeing him as a threat and others believing he can be redeemed.
Another plot could involve Harry being tricked into taking the Dark Mark. He might think he's doing something else, like a magical experiment or a ritual to save someone. But it turns out it was a setup by a dark wizard. Harry then has to face the consequences, not only the physical and magical changes brought by the Dark Mark but also the change in how others perceive him, especially his friends who might be shocked and scared.
Well, in such fanfiction, there could be a plot where Harry tries to find a way out of the forced marriage at first. He might explore magical loopholes or seek help from his friends like Hermione and Ron. However, as time goes on, he realizes that Ginny has always been there for him and starts to accept the situation. Maybe Ginny is also dealing with her own insecurities about the forced marriage, thinking Harry doesn't really want her, but through some shared adventures or conversations, they start to build a real connection.