Naruto himself could still be the main character, but this time showing even more advanced kage - level skills. Maybe he's facing a threat that only his new and improved abilities can handle.
A new character could be the main character. For example, a young shinobi who was trained in secret by an old and powerful master. He or she has kage - level powers but is unknown to the ninja world and has to prove themselves while facing various challenges and making a name for themselves in the world of kage - level shinobi.
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 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 a great main character. He has the power and the complex backstory. If he became the Kage of a different village, say the Mist Village, he could use his Sharingan and his combat skills to change the village's fate. He might try to end the long - standing blood feuds within the village using his own experiences of revenge and redemption.
The Hokage, like Tsunade or Kakashi, could be the ones training Naruto. They know Naruto well and have a lot of knowledge and skills to pass on.
It could be an original character created specifically for the fanfiction. This character might be someone who was initially not very good at ninjutsu but discovers a special affinity for the kage bunshin technique and then devotes themselves to mastering it.
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.
The Kage Fujin is likely to be one of the main characters. They are probably a powerful figure with unique abilities related to the elements or some other special power.
In a 'naruto wave kage fanfic', Naruto is almost always a main character due to his significance in the Naruto universe. The Kage are also central as they represent the leadership of their respective villages. The Wave Village might introduce characters like Tazuna or Inari as main characters too. Tazuna was important in the original Wave arc in Naruto. His skills as a bridge - builder could play a different role in the fanfic, like being a symbol of hope for the Wave Village in a time of conflict involving the Kage. Inari, with his growth and potential, could also have a major part in the story, perhaps forming an unexpected bond with one of the Kage.
Definitely Naruto. He has so much development throughout the series. If he travels back in time, he could change things like preventing the death of some of his friends or even averting some of the major disasters in the ninja world. His positive attitude and determination would make for an interesting dynamic as he tries to use his knowledge from the future to better the past.
A rogue ninja from the Whirlpool Village who resents the Kage and wants to take over the village's power. He could see Naruto as an obstacle in his path to domination, especially if Naruto is trying to help the current Kage or revive the village.