First, you need to do research on both universes. Know the rules of magic in Harry Potter and the abilities and politics in the shinobi world. Then, come up with a unique plot that ties the two together. For example, a dark force could be threatening both worlds. Decide on which characters from each side will be the main protagonists and antagonists.
It could be a wild and imaginative crossover. Picture Harry Potter characters using their magic in the midst of the chaos of the 4th Shinobi War. Maybe Hermione would be fascinated by the ninjutsu techniques and try to study them, while Naruto might be in awe of the power of the Elder Wand. It would be a clash of two very different magical worlds.
In a fanfiction scenario where Naruto starts the 4th Shinobi War, it would completely change the whole Naruto universe's narrative. Naruto is known for his desire for peace, so if he starts the war, it might be due to some extreme manipulation or a misunderstanding of the situation. For example, he could be tricked by an enemy who has disguised themselves as an ally and convinced Naruto that starting the war is the only way to protect the Leaf Village. This would lead to a situation where all his friends and comrades would be forced to either oppose him or be dragged into a war they didn't want.
One idea could be exploring the backstories of minor characters during the war. For example, how a random shinobi from a small village coped with the chaos. Another could be a what - if scenario, like what if a certain alliance was formed differently. Maybe a group of rogue ninjas decided to side with one of the major villages instead of staying neutral.
Well, it could be about a crossover where Harry Potter enters the world of shinobi. Maybe it shows Harry using his magic in a shinobi - like setting, with new adventures and challenges.
The story would take a very dark turn. Allies would become enemies, and Naruto would be seen as a villain. His friends would be divided, some might try to understand his reasons while others would fight against him immediately.
Tenten. She doesn't get much spotlight during the war. A fanfiction could explore her role in providing weapons and support from behind the scenes. Maybe she invents a new type of weapon that could turn the tide of a small battle.
In a Naruto 4th Shinobi War fanfiction, we could have a plot where the Akatsuki actually had a hidden agenda that was not related to the tailed beasts at all. They could be using the chaos of the war as a distraction to perform some other dark ritual. And perhaps a young shinobi discovers this and has to race against time to stop them while also fighting in the war.
Naruto would be an obvious choice. He played a major role in the original 4th shinobi war. But in a fanfic, you could explore different aspects of his character, like his internal struggles during the most intense battles.
In such a fanfiction, Harry would be raised in a shinobi culture. Firstly, his upbringing would be centered around the values of the shinobi, like loyalty to the village and comrades. He'd learn to move silently, use weapons like kunai and shuriken. His Hogwarts - like education would be replaced with training in the ninja arts. His friends could be other young shinobi, and they'd go on missions together. Instead of using a wand for magic, he'd use hand signs and chakra to perform amazing feats. The story could also explore how his 'shinobi - like' thinking would influence the wizarding world if he ever returned to it.