Another reason could be a sense of being wronged by the good side. If they are falsely accused of something by the Ministry of Magic or other good - aligned wizards, they might feel so betrayed that they turn to the dark side out of spite. It's a very different path from what we expect of them, but in fanfiction, all kinds of character developments are possible.
Maybe they get frustrated with the slow progress of the good side. In the fight against Voldemort or other dark forces, they might feel that the good side is too cautious and that more drastic measures are needed. This could lead them to believe that the dark side's methods are more effective.
In the wizarding world, going to the dark side often means getting involved with dark magic. Harry, Fred, and George might start using unforgivable curses. For example, they could use the Imperius Curse to control others, which would be a major departure from their moral code. This would also likely draw the attention of the Ministry of Magic, who would see them as a threat.
Another theme could be growth. Their characters develop over time, learning from their experiences. For instance, they might learn about the importance of loyalty and sacrifice through the events in the fanfiction stories.
Another theme is growth. Harry, Fred and George all grow and change throughout the series. In fanfiction, this can be further explored. For instance, how Harry's leadership skills develop as he teams up with the twins more often, or how Fred and George learn to be more responsible as they get involved in more serious matters with Harry.
One common theme is friendship. In these fanfictions, the bond between Harry, Fred and George is often explored deeply. They support each other through various challenges at Hogwarts or in the wizarding world at large.
Overcoming challenges. Given the dangerous world they live in with Voldemort around, they often have to work together to face and overcome various threats and difficulties. For example, in some fanfictions, they team up to outwit Death Eaters or solve mysterious magical problems that put Hogwarts at risk.
It's possible that there could be some strain on their relationships. The stress of being in a different time and the fear of changing things could cause arguments. Maybe they have different ideas about what they should do while in the past. For instance, Fred and George might want to have some fun and explore, while Harry is more focused on achieving a specific goal related to the future, which could lead to some friction between them.
In some fanfictions, they could be seen as soulmates in a platonic sense. Their strong bonds of friendship and the adventures they share might be interpreted as a form of soulmate - like connection.
There's a fanfiction where Harry, Fred, and George are mates and they go on an adventure in the Forbidden Forest to find a rare magical creature. Along the way, they face various challenges like dodging dangerous plants and outsmarting other magical beings. It's a really exciting read as it shows their friendship and how they rely on each other's strengths.
Maybe Harry notices some small physical differences. For example, one of them might have a slightly different hair part or a unique freckle pattern.
There's this fanfiction titled 'The Bond of Harry, Fred, and George'. It's popular because it delves into the deeper emotional connection between them. It shows how they support each other through tough times, like when Harry is haunted by Voldemort's presence. Fred and George use their humor and creativity to lift Harry's spirits and help him face his fears.