In the fanfiction, the magic railgun could be powered by a unique magical crystal. Harry, having a natural affinity for finding hidden magical objects, stumbles upon this crystal during one of his adventures. He then uses his skills learned from Dumbledore and Snape to enchant the crystal to power the railgun, which fires spells instead of normal projectiles, creating a whole new level of magical combat.
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 idea could be that Harry creates a school for magical creatures. He realizes that there are so many unique and often misunderstood creatures in the wizarding world, and he wants to give them a place to learn and grow. This school could teach them how to interact with wizards in a positive way, as well as develop their own magical abilities.
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.
The exploration of the wizarding world is a major part. When Harry creates magic, it can open up new areas of the wizarding world to be explored. It could be a hidden chamber where the new magic was born or a new group of wizards who are interested in his creations. This allows the author to expand on the already rich Harry Potter universe and take the readers on a new and exciting journey.
One idea could be exploring what would happen if Harry Potter used his magic to create a parallel Hogwarts where students from different magical cultures around the world come together. This could lead to new friendships, rivalries, and the discovery of new spells and magical abilities unique to each culture.
Harry may decide to have a more intuitive magic system. Instead of relying on wands, which can be lost or broken, the magical beings on his planet could use their minds and emotions to cast spells. The stronger the emotion or intention, the more powerful the spell. And to prevent misuse, there could be a natural balance in the planet's magic. If someone tries to use dark magic, the planet itself could resist and weaken the spell, teaching the user that dark magic is not welcome.
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.
In this fanfiction, Harry might also include some of the Hogwarts ghosts in his army. After all, they have a long - standing relationship with the school. Peeves, for example, could be a wild card in the army. He can cause chaos among the enemies. And some of the older students who have a strong sense of justice, like Cedric Diggory's friends, could also be part of it.
One idea could be a story where a new student arrives at Hogwarts with the ability to create ice sculptures that come to life. The ice creatures could either help or hinder the main characters in their adventures. For example, they could assist in a battle against the dark forces by freezing the enemies' wands. Another idea is that the ice - making magic is a lost art that Harry discovers in an old, forgotten book in the restricted section of the library. He then has to learn and master it secretly to use it against Voldemort in a final showdown.
There is also a fanfiction called 'Magic Beyond Boundaries' in which Harry creates magic that defies the traditional rules of the wizarding world. This leads to a series of adventures and challenges as he tries to convince the wizarding community that his new magic is not dangerous but rather a new way forward. The author does a great job of building a world within the existing Harry Potter universe and exploring the implications of Harry's creative magic.
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.