It could be a storage for ancient and dark artifacts. A group of dark wizards are after these artifacts stored in the cupboard, and Harry has to protect it. This could lead to an epic battle between Harry and the dark wizards right in his old home in Privet Drive.
Lightning magic could be used in duels in unique ways. Instead of the typical spells, a wizard could send out bolts of lightning that split into multiple smaller bolts to attack multiple opponents at once. Or they could create a shield made of lightning that not only blocks spells but also electrocutes anyone who touches it. This would make duels more exciting and unpredictable.
Well, think about an item like a shrinking charmed box. In a fanfiction, it could be used to store and transport large magical creatures secretly. You could also use a levitating lasso to catch flying objects or creatures. If there's a mind - reading hat in the inventory, a character could use it to gain an advantage in a wizarding duel by knowing their opponent's next move. Items can also be used in social situations. For example, a charmed perfume that makes the wearer seem more trustworthy could be used to gain information from other wizards. There are so many ways to be creative with these items, and it all depends on the writer's imagination.
They could be a canvas for new spells. A young witch might find a blank - page book that has the ability to absorb and hold new spells. She starts experimenting, writing down her ideas, and the book brings those spells to life. This could create a whole new area of magic study in the fanfiction world.
The Polyjuice Potion can be used to create a mystery. For example, a character wakes up in a different body due to the potion and has to figure out who did this to them and why. It adds an element of suspense.
Creatively, Gringotts can be a place of cultural exchange. Since it's a bank that serves both wizards and other magical creatures, it can be a place where different magical cultures interact. In a fanfic, Harry could witness a meeting between a goblin banker and a centaur who is trying to deposit some rare magical herbs as collateral for a loan. This could lead to all sorts of interesting storylines about cultural differences, misunderstandings and cooperation within the magical world.
A creative combination could be that the Central Perk coffee shop is secretly a wizarding hangout in the Muggle world. The Friends gang unknowingly serves magical drinks to wizards and witches in disguise. There could be a plot where the regulars at Central Perk start to notice strange things happening, like people disappearing into thin air or cups refilling themselves. And the Friends characters have to figure out what's going on with their new 'weird' customers.
They could be used as a form of transportation in a very unique way. Instead of broomsticks or thestrals, a wizard could enchant a swarm of butterflies to carry them. It would be a slow and more aesthetically pleasing form of travel, suitable for short distances or for a more whimsical character who wants to make a grand entrance.
One way to combine them creatively is by having an Avatar character attend Hogwarts. Imagine an Airbender learning magic, using their airbending skills to enhance their spell - casting, like using air currents to direct a spell more accurately. Another idea could be a crossover where the Avatar world's spirits interact with the magical creatures in the Harry Potter universe. For example, a Patronus could be seen as a form of spirit energy, and perhaps there could be some kind of connection or interaction between them.
One way to blend them creatively could be by having Mary Poppins visit Hogwarts. Maybe she could use her magical nanny skills to teach the students some unique lessons, like how to clean up a potion spill with just a snap of her fingers. She could also interact with the magical creatures there, perhaps taming a hippogriff in her own special way.
One way to blend them creatively is through character crossovers. For example, imagine a scenario where a wizard from Harry Potter visits the Clover Kingdom in Black Clover. The different magic systems could interact in interesting ways. The wizards from Harry Potter might be intrigued by the grimoires in Black Clover and the way magic is channeled through them. Another aspect could be the use of spells. Combining the incantations from Harry Potter with the more physical and gesture - based magic in Black Clover could lead to unique magical duels in the fanfiction.