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.
Perhaps in the fanfiction, Harry's connection to the Dark Mark leads him to discover a hidden side of Voldemort's past. This could involve uncovering secrets about Voldemort's family or the origin of the Dark Mark itself. He could then use this knowledge to try and find a way to break the connection or use it against the Dark forces in some way. For example, if he finds out that there's a certain spell that was used to create the Dark Mark and that same spell can be reversed, he might embark on a journey to find the ingredients or the incantation needed for that. And all the while, he has to deal with the prejudice and fear from those around him who might suspect him of being a Death Eater.
Some of the die - hard Death Eaters could also be antagonists. They might not trust Harry even with the Dark Mark and try to undermine him or expose his true intentions if they suspect he's not truly on their side. Bellatrix Lestrange, for example, is so fanatical about Voldemort that she would be a likely candidate to oppose Harry even if he seems to be part of the 'dark' group.
One possible plotline could be that Harry discovers some dark family secrets that lead him to embrace the dark side. Maybe he finds out that his connection to Voldemort is more complex than he thought and decides to use that power for his own gain instead of fighting against it.
One consequence could be that the Death Eaters no longer sense his connection to Voldemort. This gives Harry an advantage in his fight against them as they can't track him through the Mark.
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.
A third plotline could be that Harry's choice of electives attracts the attention of a new and mysterious teacher. This teacher has their own agenda and uses Harry's interest in the electives to manipulate him into doing things that may or may not be in his best interest. It could lead to a lot of self - discovery for Harry as he tries to figure out the teacher's true motives while also enjoying his electives.
One possible consequence is that the wizarding world would be in a state of confusion. The traditional boundaries between good and bad would be blurred, and many wizards might not know how to react. Some might fear Harry's new - found power over the Dark.
One possible plotline could be that Fenrir takes Hermione as a hostage to use against the Order of the Phoenix. He might think that her connection to Harry Potter makes her a valuable bargaining chip. Another could be that he is under the control of a dark wizard who orders him to capture Hermione for some dark magic ritual. In some fanfictions, it could also be that Fenrir has some sort of personal vendetta against Hermione and takes her to exact revenge for something she did to him or his kind in the wizarding war.
One plotline could be that Harry is influenced by the Dark Lord's ideology from an early age. Maybe he discovers some 'truths' as he sees them that make him believe the Dark Lord's way is the right one. For example, he might think that the wizarding world's current order is corrupt and only the Dark Lord can fix it.
In some fanfictions, Harry might use powerful ancient magic that he discovers in a hidden library or from an old wizard. This magic could be specifically designed to counter the dark magic of the Dark Mark.