Perhaps Harry uses some powerful ancient magic he discovers. Maybe he finds a spell in a long - forgotten book in the Room of Requirement that has the power to break dark enchantments like the Dark Mark.
It could change the way the wizarding world views Snape. Since the Dark Mark is a symbol of his connection to the Death Eaters, its removal might make people see him as a redeemed figure. For example, the students at Hogwarts who once feared him might start to respect him more. And the Order of the Phoenix members might trust him even more fully.
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.
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.
The Dark Mark on Snape in fanfiction can be a key plot device. It might be used to create tension in a story. For example, if other characters discover it, they may question his true allegiance. Writers often explore how Snape deals with the stigma associated with the mark. He could be shunned by some in the wizarding community because of it, and this can lead to interesting character development as he tries to prove himself.
In this fanfic, Harry might have discovered a powerful magic spell or potion to achieve it. Maybe he found some ancient knowledge that gave him the means.
In fanfiction romances, Severus Snape's Dark Mark can be a source of angst. His love interest may initially be repelled by it, seeing it as a sign of his dark past. But as the story progresses, they might come to understand the mark as a part of him that he's trying to overcome, which deepens their relationship.
It changes his character development in that he becomes more distrustful. Since he has the Dark Mark, he might start to doubt the intentions of those around him. He might think that people will always suspect him, so he becomes more withdrawn. Also, his sense of self - identity is shaken. He's no longer just the Boy Who Lived, but now he's someone with a mark associated with the darkest of magic. This could lead to him exploring his own morality more deeply. He might question if he's doomed to be like Voldemort just because of this mark, and he has to fight against that internal struggle to define who he really is.
Yes, there are likely some out there. Fanfiction often explores unique and unexpected relationships. Some writers might be intrigued by the idea of Harry being Snape's son and delving into the 'dark side' aspects could involve exploring Snape's more complex and perhaps morally ambiguous nature, as well as how that would impact Harry's character and his journey in a different way than in the original story.
The plot would change drastically. Firstly, Harry would lose the trust of many. Dumbledore might be more suspicious of him. Secondly, the Death Eaters could have different plans for him. They might try to use him as a weapon against the Order. Thirdly, Harry would have to constantly hide the mark, adding a new layer of secrecy to the story.
Snape might become more empathetic. After seeing Harry's memories, he could understand the pain and pressure Harry has been under. This new understanding might make him more protective of Harry in some way.