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.
Well, if they go to the dark side, their relationship with their families would be severely strained. The Weasley family has always been against the dark arts, and Fred and George's actions would be a huge disappointment. Harry, who has always had a special bond with the Weasleys, would also be seen as a traitor by them. This could lead to a lot of family drama and heartbreak.
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 this fanfiction, Fred and George might get into all sorts of mischief with the Marauders. They could learn some new pranking tricks from James and Sirius. Maybe they'd team up to play a huge prank on Snape, just like the Marauders used to do.
If they joined Voldemort, the story would go off in a very different direction. Harry might be able to learn more about the Dark Arts directly from Voldemort, which could either corrupt him completely or give him some unique knowledge to use against Voldemort later. Fred and George would likely face a moral struggle. Their loyalty to their family and friends would be tested, and they might try to find ways to protect those on the side of good while seeming to be on Voldemort's side in the fanfiction.
A simple plot could be that there is a lemon - themed magical competition at Hogwarts. Harry teams up with Fred and George to participate. They have to create the best lemon - based potion or spell. They struggle at first as they can't find the right ingredients related to lemons, but then they discover a secret stash of rare lemon - scented herbs in the Room of Requirement which gives them an edge in the competition.
Well, they could get into all sorts of mischief. Maybe Fred and George would introduce Hikaru and Kaoru to some of their wizarding pranks. The twins might be fascinated by the unique bond between Hikaru and Kaoru and try to figure out if there's some sort of magic in it. And Hikaru and Kaoru could be equally intrigued by the magical world of the twins and ask them about all the cool spells and potions.
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.