It could be a rogue ninja who has a vendetta against all the villages represented at the summit. He might have been wronged in the past by the combined actions of the five great villages and now seeks revenge. He could use the chaos of the summit to launch attacks and try to undermine the stability of the shinobi world.
Maybe an ancient evil spirit that has been awakened by the gathering of so much chakra and power at the summit. This spirit could possess one of the Kages or their attendants and start causing havoc, trying to break the fragile peace among the villages. The other Kages would then have to figure out how to deal with this unseen and powerful enemy.
It could be Naruto himself. Since he has a connection to all the villages in some way, he could play a crucial role in the fanfic, perhaps as a mediator or someone who uncovers a hidden conspiracy during the summit.
Sasuke could be one to stop Naruto. They have a complex relationship and Sasuke might understand Naruto's motives and be able to talk him down.
In this fanfiction, Naruto might be pushed to attack the Five Kage Summit due to some misinformation or a threat he perceives to Konoha. Maybe he thinks the Kages are planning something against his village.
In a 'naruto five kage summit fanfiction', a great plot could be that the Mizukage reveals she has evidence of a traitor among the other Kages' entourages. This leads to a tense investigation during the summit, with accusations flying and old grudges resurfacing. The story could explore how the Kages balance their trust in their own people and the need to maintain diplomatic relations during such a crisis.
Naruto could still be the main character. As the Hokage, he would be at the center of the Kage Summit. His ideals of peace and unity would play a major role in the interactions with the other Kages. He might try to convince the other Kages to support a new initiative for a more united ninja world.
A new, young ninja from a minor village could be the main character. They could be trying to make a name for themselves and their village at the Kage Summit. They might get involved in some political intrigue or even a fight between the Kages and prove their worth.
It could be Danzo. He has always been a shadowy figure with his own agenda. He might see Naruto as a threat to his control over the village and engineer his banishment.
Perhaps in the fanfic, a young shinobi manages to infiltrate the Five Kage Summit and has a significant impact on the discussions. This shinobi could bring new ideas or warnings that change the course of the kages' decisions. For example, he might have uncovered a plot by a rogue ninja group to take advantage of the kages being gathered in one place. This would not only create tension between the kages and the external threat but also among the kages themselves as they debate how to handle this unexpected situation.
The Kages themselves are often main characters. For example, Tsunade from Konoha, Gaara from Suna, and the other village leaders. They play important roles in the political and power - related discussions during the summit.
Other villages' leaders or powerful ninjas who don't believe Naruto is ready to be a Kage. They might try to test him or even plot against him during diplomatic meetings before the summit, seeing his new position as a potential weakness in the balance of power.