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.
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 a magical crossover where a spell goes wrong in the Harry Potter world and somehow links to the Avengers universe. A male character from either side could end up pregnant due to some strange magical - scientific mishap. For example, if Loki were to be affected by a Hogwarts spell that causes mpreg. It could lead to a lot of chaos as the Avengers try to figure out what happened while the Harry Potter characters deal with the consequences of their spell reaching another dimension.
Then, think about the characters' reactions. For example, how would Harry and his friends react to Loki in this situation? How would Loki himself handle it? You can also explore how the different magical cultures in both universes view this situation. In the Harry Potter world, it might be completely unheard of, while in Loki's world, there could be different attitudes.
A third idea could be that Harry and Clint are both transported to an alternate dimension where they have to find their way back. Along the way, they meet various strange creatures and characters. Clint uses his survival skills and archery to fend off threats, while Harry uses his magic to solve puzzles and open portals that might lead them home.
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.