A possible plot is a friendly competition between them. For example, they could have a race using different magical means of transportation around Hogwarts and the surrounding areas. Along the way, they encounter various magical creatures and obstacles that they have to deal with. This could also lead to some character development as they learn from each other's strategies.
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.
In some fanfictions, an interesting plot point might be that Fred and George form a sort of 'protection squad' for Harry. They recruit other students who also care about Harry and together they create a network of protection around him. This could lead to some really exciting adventures as they try to outwit the Death Eaters or other enemies. Another aspect could be that they help Harry with his studies in a very unorthodox way, using their own brand of magic and knowledge that the professors might not approve of, but which is effective for Harry.
In a more adventurous fanfiction, Harry, Fred, and George are on a quest to find a lost lemon - colored magical creature. This creature is said to be able to grant wishes. Along the way, they face many obstacles like dark magic traps and unfriendly magical beasts. They rely on their combined skills and the power of friendship to keep going on their search.
There might be a fanfiction where Fred and George use their magical skills to create a new sport. They combine elements of Quidditch with some Muggle extreme sports. Harry joins in as a tester, and they all have to face challenges from the Ministry of Magic who think the sport is too dangerous. In this story, we could see their creativity and Harry's loyalty to his friends as they fight to keep their new sport alive.
One interesting plot could be Harry helping Fred and George start a new, even more successful business. He might use his fame and fortune to back them up. Another could be Harry teaching them advanced magic he learned from his adventures. It would be really cool to see how they incorporate that into their pranks and inventions.
Another plot point might be that the twins are actually quite amazed that Harry can tell them apart. This could lead to a closer bond between Harry and the twins, and they start to involve him more in their secret schemes because they trust him with this special knowledge.
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.
One idea could be a fanfiction where Hermione helps Fred and George start a magical business outside of Hogwarts that involves using her academic knowledge in a practical way. Maybe they create a new type of spell - bound product together.
Well, a common plot is the OC having a secret that the twins accidentally discover. Maybe the OC is a Metamorphmagus and is trying to keep it under wraps, but the twins notice something strange and start investigating. This leads to a series of events where the three of them bond over the secret. Another plot could be that the OC is in a difficult situation, like being bullied by some Slytherins, and the twins come to their rescue. Then, a friendship and potentially more develops. There are also plots where the OC has a special mission or quest, and the twins join in to help, creating an adventure - filled story.
One plot idea could be that George and Lennie find a hidden treasure on the ranch. They have to keep it a secret from the other workers who might be jealous and try to steal it. Another idea is that they rescue an injured animal on the ranch and nurse it back to health together. It shows their kind hearts.