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.
One idea could be focusing on a minor character's perspective during the war. For example, a young genin who is caught in the middle of the chaos. They could witness the great battles between the powerful shinobi and have their own journey of survival and growth.
The unique perspectives. For example, if it shows the war from the point of view of a civilian or a minor character. This can give a fresh look at the events that we are used to seeing from the main characters' eyes. Also, exploring the backstories of the characters involved in the war can make it interesting.
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.
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.
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.
One great fanfic could be 'The 4th Shinobi War: Unseen Perspectives'. It might offer unique views of the war that the original series didn't fully explore. Another could be 'Among the Chaos of the 4th Shinobi War', which might focus on the experiences of minor characters during the war. And '4th Shinobi War: Alternate Realities' could present different outcomes of the war based on various what - if scenarios.
'The Unseen Hero of the 4th War' is a great one. It focuses on a character who is often overlooked in the original story but plays a crucial role in the fanfiction. The writing style is engaging and it really dives deep into the character's emotions during the chaos of the war.
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.
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.