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.
The plot could develop in a number of ways. For one, Voldemort might use Harry as a symbol to further his control over the wizarding world, claiming that even the 'Chosen One' has seen the error of his ways and joined him. Fred and George could be used to spread fear and chaos with their magical skills, but they might also try to use their position to gather intelligence for the resistance. Another aspect could be the reaction of the other characters. Hermione and Ron would be devastated and likely try to find a way to rescue their friends. The Order of the Phoenix would have to reevaluate their strategies completely.
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.
In such a fanfiction, Harry might be raised with a very different set of values. Voldemort could try to mold Harry into his own image, perhaps teaching him dark magic. But Harry might also secretly hold on to his own sense of good. For example, he could use the knowledge Voldemort gives him to ultimately turn against him.
In such a fanfiction, Harry might become a very different person. He could be more in tune with the Dark Arts. Maybe he would have a skewed sense of morality, influenced by Voldemort's views. He might also be more powerful in the Dark Arts but struggle with his identity as he is constantly in conflict between what he's been taught by Voldemort and his own inner goodness.
It would be really interesting. Fred and George are full of mischief and fun. They'd likely teach Harry all their pranks and tricks. Harry would grow up in an environment filled with laughter and inventiveness. He might be more light - hearted and have a different sense of humor compared to the original story.
If they joined Voldemort, Harry might lose some of his natural courage. He'd be in a world where his usual values are flipped, and this could make him seem more hesitant. Hermione would probably have to hide her intelligence more. Voldemort doesn't like those who are too smart or independent. So she might act more subservient in public, but in private, she'd be devising plans to break free and expose Voldemort's true nature.
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.
Harry's sense of justice would be a strong factor in their interactions. Fred and George, with their love for fun and rule - breaking (in a harmless way mostly), would constantly test Harry's limits. Rance could be the one who comes up with new and unexpected ideas that either bring them all closer together or cause some temporary rifts. Their personalities would create a dynamic where there's always something interesting going on, whether it's a friendly argument, a joint adventure, or a discovery of a new magical concept through their combined viewpoints.
If Harry gives Voldemort the stone in fanfiction, it could be a very dark turn of events. Voldemort would likely use the stone to bring back his physical form more fully or perhaps gain some sort of immortality. However, some fanfictions might explore the idea that this act actually unites the wizarding community against Voldemort in a way that they hadn't been before. They might see Harry's sacrifice as a call to arms and fight back even more fiercely.
Well, it could be really strange. Harry might find some new perspectives on his enemies or himself. And Voldemort could get angry as he might not like how he is portrayed in some fanfictions. Maybe he'd try to change things according to what he reads.