In a Harry Potter and Old Gods fanfiction, the story could start with a strange event that links the two worlds. Say, a group of wizards discovers an ancient artifact that has a connection to the Old Gods. As they explore this connection, they find that the Old Gods are not as removed from their world as they thought. The Old Gods could then start to influence the fate of the characters. For instance, Harry Potter himself might be chosen by one of the Old Gods for a special mission, which could take him to places beyond the known wizarding world and into the realms of the Old Gods. This would expose him to new challenges, different cultures, and powerful magic that he has never encountered before.
Well, in such a fanfiction, the Old Gods could have a significant impact on the magic system. They might introduce a new type of magic that is more primal and different from the traditional magic in Harry Potter. The characters would then have to adapt to this new form of power. Also, the Old Gods could bring their own rivalries and politics into the wizarding world, causing chaos or new alliances among the wizarding community.
One interesting story could be where Harry Potter discovers he has some Greek god heritage. Maybe his magical abilities are actually a gift from a Greek god, like Athena. This could lead to him having to learn about both the wizarding world and the world of Greek gods, and how to use powers from both. It would be a really cool mash - up of the two universes.
One great example could be a story where a wizard from the Harry Potter world discovers ancient Valyrian magic spells. This could lead to all sorts of interesting plot twists, like using the powerful Valyrian magic to combat dark wizards in the Harry Potter universe. Another could be a crossover where a character from the Valyrian era is transported to Hogwarts and has to adapt to the different magical system there.
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 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 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.
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 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.
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 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.