Perhaps in the fanfiction, Harry Potter somehow travels to the Supernatural universe and meets Gabriel there. Gabriel then decides to help Harry find a way back home while also showing him the strange and dangerous aspects of the Supernatural world.
A fanfiction could involve Gabriel getting involved in the fight against Voldemort. He might use his trickster abilities to outwit Voldemort and his Death Eaters. For example, he could create illusions that confuse the enemy, which would give Harry and his allies an advantage in the battle.
One idea could be that Loki, being the mischievous character he is, finds his way into the world of Harry Potter and Supernatural. He could start causing chaos in Hogwarts by disguising himself as various characters. In the Supernatural world, he might team up with the Winchester brothers for a short time, but then double - cross them for his own amusement.
One idea could be that Selene finds herself magically transported to the wizarding world of Harry Potter. She might initially be confused but then use her skills as a vampire to help in some sort of battle against dark wizards. For example, her enhanced speed and strength could give the Order of the Phoenix an edge in a fight.
One idea could be that Harry Potter, as an Animagus, somehow gets transported to the Marvel universe. He could team up with the Avengers in his animal form, perhaps using his unique abilities to gather intelligence. For example, as a stag, he could move quickly and unnoticed in certain terrains, providing valuable information to the Avengers before a battle.
Well, Gabriel could be a key figure in helping Harry Potter understand some of the deeper mysteries of magic. Since Gabriel has a different kind of power in the Supernatural world, he could show Harry that there are other ways to use magic that are not taught at Hogwarts. For instance, he could teach Harry how to use magic to manipulate time on a small scale, which could be useful in Harry's fight against the Dark Arts. And Harry, in turn, could introduce Gabriel to the rich magical culture and history of the wizarding world, like the story of the four founders of Hogwarts or the origin of the Hallows.
One idea could be that Fluttershy somehow gets transported into the Harry Potter world. Maybe she arrives at Hogwarts as a new kind of magical creature that the students have never seen before. Her gentle nature might initially cause confusion among the more rowdy students, but she could gradually make friends with the likes of Luna Lovegood, who would be intrigued by her uniqueness.
Another concept could be that Harry Potter discovers he has a connection to Magneto's mutant powers. This might happen during a strange encounter with a dark magic object that unlocks a latent power within him. He then has to learn how to control these new abilities while also dealing with the threats in his own wizarding world, and he might seek out Magneto for guidance in a secret meeting across the boundaries of their two worlds.
One idea could be a crossover where Kakashi is summoned to the Harry Potter world by a dark magic spell gone wrong. He then uses his ninja skills and Sharingan to navigate the wizarding world, perhaps even teaching some Defense Against the Dark Arts classes with his unique combat knowledge.
Maybe in a fanfiction, Harry Potter and his friends accidentally stumble upon a hidden Cybertronian relic. This relic has some sort of power that attracts Megatron. Harry and his friends then have to defend the relic from Megatron while also trying to figure out what it does and how to use it to their advantage in the fight against Voldemort.
One idea could be a crossover where Harry Potter accidentally travels to the One Piece world and meets Whitebeard. He could learn about their different forms of magic and power, and together they face a new threat that combines the dark magic from his world and the powerful pirates in the One Piece world.
One idea could be a crossover where Harry Potter accidentally ends up in the world of Ranma 1/2. Maybe he's on a magical experiment gone wrong and lands in Nerima. He could use his magic to help Ranma deal with his curse, like creating a spell to keep him from changing into a girl when splashed with cold water.