Well, they could change a lot of things. Maybe Naruto would prevent some of the tragedies that happened in the past. He could stop the Nine - Tails attack on the village, which would completely change his own life and the lives of many in Konoha.
Well, Kurama might train Naruto by teaching him how to better control and access chakra. Since Kurama has a vast amount of chakra, it could show Naruto unique ways to harness and use it. For example, it could guide Naruto in creating more powerful and complex jutsu by using their combined chakra in a more efficient way.
He could meet his past self. It would be really interesting to see how he interacts with his younger, more naive self. He might try to give advice to his past self, like how to better control his power or avoid certain dangerous situations.
Obviously, Naruto and Kurama are going to be main characters. Since it's about Kurama going back in time, he'll be central to the story. Naruto will also be important as Kurama's jinchuriki and the one whose life Kurama is likely to impact. Minato might also be a significant character as he was Naruto's father and had a major role in the original story. And there could be appearances from other key characters like Sasuke, as Naruto's relationship with him was a big part of the original Naruto series.
One interesting plot could be Kurama using his knowledge from the future to prevent certain tragedies in Naruto's past. For example, he might stop Orochimaru's early attacks on Konoha more effectively. Another plot could be Kurama helping Naruto train earlier and more efficiently, leading to Naruto becoming much stronger at a younger age. And perhaps Kurama could also change some of the relationships within the village, like improving Naruto's standing among the villagers much sooner.
The Kurama Brothers could be entities that have a similar origin to Kurama. They might have been part of the same chakra - based creation in the Naruto universe. They could possess some of Kurama's traits like great power and a connection to the natural energy in the world. They are often depicted as having their own personalities, with one being more hot - headed and the other more calm and calculating. Their relationship with Naruto usually involves them using their powers and knowledge to help Naruto grow as a ninja and a person. They might also have their own goals and motives that are intertwined with Naruto's journey.
Kurama makes a comeback in the manga. The return is well-plotted and adds an interesting twist to the story. It also ties up some loose ends and brings new possibilities.