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.
In a more complex fanfiction, the concept of different realms in all three universes could be explored. Loki, who has knowledge of different realms, could guide characters from Harry Potter and Supernatural through a journey across these realms. For example, the characters might have to find a magical artifact that exists in all three universes to stop an impending doom that threatens to destroy all their worlds. Harry Potter characters could use their spells, the Winchester brothers their hunting skills, and Loki his trickery to achieve this goal.
Perhaps a fanfiction where Loki is sent to the Harry Potter world as a form of punishment. There, he encounters a powerful magical artifact that causes the mpreg situation. The story could then revolve around how he copes with this, his relationships with the Harry Potter characters, and how they all react to this very strange event that has no precedent in either of their worlds.
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.
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.
Another concept could be a crossover where the magic in the Harry Potter universe has an unexpected effect on Android 18. For example, it could start to interact with her android systems in strange ways, giving her new powers or abilities that are a combination of magic and her android technology. This would lead to a whole new set of adventures as she tries to figure out what's happening to her.
One idea could be that Gandalf somehow gets transported to the Harry Potter universe. He could become a mentor to Harry, using his vast magical knowledge and wisdom from Middle - earth. Maybe he helps Harry understand some ancient magic that even Dumbledore wasn't fully aware of.
One idea could be that Hawkgirl somehow gets transported to the Harry Potter universe. Maybe she arrives during a Quidditch match and her flying skills astound everyone. She could then join forces with Harry and his friends to fight against the dark forces in Hogwarts, using her Nth metal mace and flight abilities in unique ways.
One idea could be that Loki gets turned into a vampire while in the midst of his schemes against the Avengers. This new state gives him different powers and motives. He might struggle with his newfound thirst and also use his vampire abilities to outwit the Avengers in different ways.
One idea could be that Loki, being the mischievous character he is, finds his way into the wizarding world of Harry Potter. He could start causing chaos in Hogwarts, perhaps by disguising himself as a student or a professor and playing pranks that are far more elaborate than those of the Weasley twins. For example, he might turn the entire Great Hall into an ice rink during breakfast time.