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.
A plot could revolve around a shinobi who discovers a hidden power or ancient jutsu during the 4th shinobi war. This new power could change the course of the war but also put the shinobi in a difficult position as others may try to steal or control it for their own purposes. It could be a story of power, responsibility and the choices one has to make in a time of war.
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.
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.
One idea could be to focus on a young, unknown shinobi who rises to prominence during the war. Maybe they discover a new jutsu that changes the tide of a crucial battle.
A third idea is a story centered around a shinobi who was thought to be dead but actually survived and is now trying to get back to the battlefield. Along the way, they discover secrets about the origin of the war or new jutsu that could turn the tide of the battle. They have to overcome their own physical and mental weaknesses while also evading the enemy who believes them to be dead.
A really cool plot could be an ashikabi who has the ability to communicate with the spirits of past shinobi. They use this power to gain ancient knowledge and techniques that no one else in the present day has. However, this power also attracts the attention of dark forces who want to use it for their own evil purposes, leading to a battle between good and evil in the fanfic.
One idea could be to focus on a young and inexperienced ninja who gets caught in the middle of a major battle during the Third Shinobi War. They could witness the horrors of war and have to quickly learn to survive. Maybe they form an unlikely alliance with a ninja from the enemy side who also has a different view on the war.
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.
One idea could be to explore the backstories of minor characters during the war. For example, how the war affected the normal villagers who were not ninjas. Another could be a what - if scenario where a different alliance was formed among the shinobi villages.
One idea could be focusing on a young, unknown shinobi who rises to prominence during the war. They might have a unique jutsu that catches everyone's attention.
One idea could be focusing on a young and inexperienced ninja who gets thrust into the chaos of the Third Shinobi War and has to quickly learn to survive. Maybe they form an unlikely alliance with a ninja from an enemy village because they both realize the senselessness of the war at a personal level.