Another plot could be that Draco's sorting into Gryffindor causes a rift in his family. His parents are disappointed, but Draco finds a new family among his Gryffindor housemates. He could also have a love interest in a Gryffindor girl, and they have to overcome the house rivalries to be together. The story could explore how Draco changes and grows in this new environment.
One idea could be that Draco, being in Ravenclaw, uses his intelligence to re - evaluate his family's pure - blood ideology. He might start to question the old ways and form new friendships with Muggle - borns or half - bloods in Ravenclaw. Another could be that he becomes a rival to Hermione in academics, leading to a different kind of relationship between them than in the original series. He could also be part of a group in Ravenclaw that is dedicated to researching ancient magic and discovers something that changes the wizarding world.
One idea could be that Hermione's thirst for knowledge takes a different turn in Ravenclaw. Instead of focusing mainly on academic success as she did in Gryffindor, she delves deeper into the more esoteric and theoretical aspects of magic. For example, she might start exploring ancient runes and their hidden meanings, which could lead her on adventures to discover lost spells.
One idea could be Hermione using her intelligence to change the Slytherin house's reputation from within. She could start study groups that include students from all houses, gradually breaking down the traditional rivalries. Another plot could involve her befriending some of the more misunderstood Slytherin students and together they work on a secret project that benefits the whole school.
One idea could be that Regulus Black, despite being a Slytherin in the original story, has a secret admiration for Gryffindor values. So, in the fanfiction, he could start secretly taking part in Gryffindor - like adventures. For example, he might help Gryffindor students in a fight against some dark forces in the Forbidden Forest.
Well, at first Harry would be very suspicious of Draco. But as Draco shows his Gryffindor side, Harry might start to trust him more. They could become reluctant allies, especially when facing a common enemy.
One idea could be Harry having to deal with the prejudice from his Gryffindor friends. For example, Ron might be initially very suspicious and distant, which forces Harry to prove that he's still the same person. Another plot could involve Harry using Slytherin's more cunning and ambitious traits to fight Voldemort in a different way than in the original story. He might form alliances within Slytherin that were previously unthinkable.
One plot idea could be that she struggles with the family expectations of being a Malfoy while having the brave and chivalrous values of Gryffindor. Maybe she has to keep her Gryffindor identity a secret from her family at first. Another could be that she forms an unlikely friendship with a pure - blood supremacist in Slytherin and tries to change their views through her Gryffindor - influenced actions. And she might also be involved in a quest to protect Hogwarts from a dark force, using her unique position of being a Malfoy yet having Gryffindor qualities.
A plot could be that a Gryffindor Animagus accidentally reveals their form during a Quidditch match. This causes chaos as everyone tries to figure out who it is. It also draws the attention of the Ministry of Magic who have strict rules about unregistered Animagi. The character then has to deal with the consequences while also trying to prove they mean no harm.
One interesting plotline could be Hermione struggling to fit in Slytherin at first. She's used to the values of Gryffindor, so being in Slytherin where ambition and cunning are prized in a different way would be a big adjustment. For example, she might have to deal with the initial suspicion from her Slytherin housemates who are not used to having a 'Mudblood' among them. But as time goes on, she could use her intelligence to show them that she belongs there and maybe even change some of their views on blood purity.
In fanfiction, if Snape was sorted into Gryffindor, he might have had a very different relationship with the Marauders. Instead of being their rival from Slytherin, he could have been a part of their group. He may have been more open about his emotions and not as bitter as he was in the original story. His relationship with Lily might also have developed in a more positive way without the house rivalry constantly straining their friendship.