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.
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.
Obviously Naruto would be one of the main characters as the title implies he's the one attacking. And the Kages, like Gaara, Tsunade, and the other village leaders would also be main characters since they are at the summit that Naruto is attacking.
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.
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.
One possible plot point could be Naruto's reasons for attacking the Kage Summit. Maybe he's been misinformed or is trying to prove a point. Another could be the reactions of the various Kages, like how Tsunade might be conflicted because she knows Naruto.
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.
Gaara could be one of his potential allies. Since they have a history together and Gaara has changed a lot, he might support Naruto. Also, the Raikage might become an ally if Naruto can prove that his intentions are good for the entire ninja world.
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.
Perhaps in the fanfiction, there are internal power struggles within the villages that are revealed at the Kage Summit. A young ninja from one of the villages might try to influence the decisions of their Kage during the summit. There could also be a cultural exchange aspect, where the Kages learn about each other's villages' traditions and find ways to incorporate beneficial elements into their own villages.