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.
In a fanfiction where Naruto starts the 4th Shinobi War, the story would progress in a complex way. Naruto's actions would force the other shinobi to reevaluate their own beliefs. Some might start to question if there is some truth in what Naruto is doing. His friends like Sakura and Kakashi would be heartbroken. They would try to reach out to Naruto and find out what's wrong. Meanwhile, the other villages would be plotting to take down Naruto. But Naruto being Naruto, he would have some kind of plan or reason behind his actions that would gradually be revealed as the story unfolds.
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.
The story would likely shift focus to Naruto's actions. He'd probably start by assessing the situation of the war and then use his abilities to start making a difference.
In a Naruto x Ino after - war fanfiction, the story might progress as they both heal from the traumas of the war. Ino could be involved in helping Naruto deal with his nightmares, using her knowledge of the mind. Meanwhile, Naruto could inspire Ino to be more confident in her abilities outside of her family's traditional role. They could attend village festivals together, and slowly, their friendship would turn into love. They would also face some obstacles, like the opinions of their friends or their own insecurities, but they would overcome them as they grow closer.
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.
If Naruto was born before the Second Shinobi War, the story would be drastically different. For one, the political landscape of the ninja world at that time was very different. There were different power struggles and alliances. Naruto might have grown up in a more chaotic and war - torn environment. His presence could have potentially changed the outcome of some of the battles during the Second Shinobi War. Maybe he could have influenced some of the key players in a way that would lead to a different future for the ninja villages.
Sasuke would likely be conflicted. On one hand, he and Naruto have a deep bond, but on the other, Sasuke has his own ideas about justice. He might try to stop Naruto at first, reasoning with him. If that fails, he could be forced to choose a side, either to support Naruto's cause if he believes there's some merit to it, or oppose him to protect the shinobi world as he knows it.
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's Rasengan would be a game - changer. In the Second Shinobi War, such a powerful and unique jutsu would be unheard of. He could use it to take out enemy strongholds or important leaders quickly.
He might change the course of many battles. For example, he could use his Rasengan to take out powerful enemies quickly. His ability to inspire others would also boost the morale of the shinobi on his side.
His powers would be seen as extraordinary. The Rasengan alone would be a very powerful and unheard - of jutsu at that time.