Perhaps he invents a spell called 'Transfigura-Shield'. This spell combines transfiguration and shielding magic. When cast, it can transform any nearby objects into a protective shield. For example, it could turn a pile of rocks into a solid stone wall to block incoming attacks.
Harry could also learn from some of the more experienced and eccentric professors at Hogwarts. For instance, a professor who specializes in experimental magic might guide him in the process. They could work together in a secret laboratory, testing different combinations of wand movements, incantations, and magical ingredients.
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.
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.
Well, it could be about Harry using his skills and magic knowledge to custom - create a super - powered Firebolt. Maybe he adds some unique enchantments to it for speed or stealth.
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 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.
Dumbledore, even if in spirit or through some magical means, could be an ally. His wisdom and foresight could guide Harry in making important decisions for the kingdom. Also, the house - elves, if Harry treats them fairly and gives them freedom and a place in the kingdom, could be loyal allies. They could take care of the day - to - day running of the castle or important buildings in the kingdom, using their unique magical abilities.
One spell Harry could create is a 'Lumos Maximus Amplificatus' which would create an extremely bright and long - lasting light, much stronger than the regular Lumos spell. This could be useful in very dark and dangerous places like the Forbidden Forest at night.
In some fanfiction, the Engorgio spell is used in really creative ways. For example, it might be used to enlarge a hidden passageway to make it easier for characters to escape. Sometimes it's used on magical creatures to make them seem more intimidating during a battle.