One interesting railgun fanfiction could be one where the characters go on a new adventure outside of their usual Academy City setting. For example, they might end up in a parallel world where the rules of their powers are different.
Another way is to join railgun - related forums or communities. There, fans often share their favorite fanfictions or even write their own. You can get recommendations directly from other enthusiasts who have similar tastes.
One idea could be that Harry discovers an ancient spell book in the Room of Requirement that has the incantations for creating a magic railgun. He might use it to defend Hogwarts against a new, powerful dark force that is immune to traditional magic spells.
One interesting 'Harry Potter Railgun fanfiction' could be a story where the magical world of Harry Potter intersects with the scientific abilities in Railgun. For example, Hermione might meet Misaka Mikoto and they exchange knowledge about magic and electricity.
Another theme is friendship and loyalty. The speedster usually has to interact with the other characters from 'A Certain Scientific Railgun'. These interactions often revolve around the concepts of friendship and loyalty. They might have to choose between helping their friends or using their speed for personal gain, which makes for an interesting story arc.
He could start by using his knowledge of transfiguration. For example, he could transfigure normal metals into a special alloy that can conduct both magic and electricity, which is essential for a railgun. He might also use his Patronus charm as a guide or source of power. The positive energy of the Patronus could be harnessed to create a powerful force within the railgun.
A frequent theme is the crossover of their worlds. How would Izuku fit into the world of Academy City where A Certain Scientific Railgun is set? Writers like to play with the idea of him adapting to the different rules and technology there. Also, the theme of facing challenges together is common. They might have to battle a common enemy or overcome some sort of prejudice between their two groups of friends.
Another place to check could be some dedicated Harry Potter or Railgun fan forums. Fans often share their own works or recommend good fanfictions they've read on these forums. Just do a search for relevant forums and see what you can find.
Some fanfictions might have a plot where the relationship between the characters is explored in a new light. For example, focusing on a friendship that was only hinted at in the original, like how Mikoto and Kuroko's friendship could be tested in a unique situation that forces them to reevaluate their bond.
Mikoto Misaka is definitely one of the most popular. She's the main character in the original work, and in fanfiction, people often like to expand on her story, her relationships, or even give her new powers or challenges.