It's hard to say precisely without more context. Generally, fanfiction based on a '7th stand user' might be about exploring uncharted aspects of that character. For example, if it's part of a larger fictional universe with stands, the fanfiction could be about the 7th stand user's origin story, their battles, or their inner thoughts and feelings. It could also be a crossover with other elements from the same or different fictional universes.
One idea could be a crossover between Jojo's Bizarre Adventure stand users and the characters from Naruto. For example, imagine a stand user having to face off against ninjas in the Hidden Leaf Village. Another could be a crossover with the world of Harry Potter, where stand users attend Hogwarts and their stands interact with magic. Maybe a stand user's ability could be related to a particular spell or magical object.
You can try looking on fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. These platforms usually have a wide range of fan - created content, and there's a good chance you'll find '7th stand user fanfiction' there.
Another concept might be an OC with a Stand that has the ability to manipulate time. In the DxD world filled with battles and power struggles, this could be a game - changer. The OC could use it to freeze time during combat, giving them an edge, or to reverse time to undo a harmful spell or attack.
First, decide which universes you want to cross over. For example, if it's Jojo's Bizarre Adventure and a superhero universe, figure out how the stand users and superheroes will interact. Then, create a basic plot. Maybe the stand users are brought into the superhero world to help fight a common enemy. Next, develop your characters. If a stand user meets Superman, how will they react to each other's powers? Start writing simple scenes and build from there.
Give your OC a unique goal or motivation. In DxD, there are many power - hungry characters. But your OC could be different. Maybe they want to use their Stand to protect the innocent or to find a way to bridge the gap between different supernatural factions. This will make their actions and decisions in the fanfiction more interesting and distinct from other characters.
Maybe it could be about a new character who is a true ice user and enters the Naruto world. This character might start as an outcast because of their strange power but then prove themselves in battles against various villains.
Ordinary users could apply for "Heaven's Wings Audible Reading" as long as they had a mobile phone or tablet and had signed a contract with the relevant reading platform.
A magician could stop by the lemonade stand. He tries to do a trick with the lemonade, making it 'disappear' but actually just spills it all over himself. Everyone at the stand starts laughing, and the magician ends up buying a cup to make up for it. He also gives a little magic show for free to the kids at the stand. It becomes a really memorable and funny event for them.