It could be a new character, perhaps someone from a far - off land who has a grudge against Naruto's future self for some reason. This character might be extremely powerful and has plans to destroy everything Naruto holds dear. When Naruto sees this future, he has to find a way to stop this new threat before it even begins to form.
It could be a dark ninja who also has the ability to manipulate spirits. He might use this power to create chaos in the ninja world and Naruto has to stop him while also dealing with the spirits he sees.
It could be a rogue ninja who was once a student of Orochimaru and has now developed his own dangerous jutsu. He wants to take over the ninja world and believes the old ways are holding everyone back.
Maybe in the fanfic, Naruto sees a future where Konoha is destroyed. This makes him determined to change the future. He could start by strengthening the village's defenses, forging new alliances, and also trying to figure out the root cause of the destruction in the future he saw. He might also try to uncover any hidden threats within the village that could lead to such a catastrophe.
Well, perhaps Naruto sees the future where his friends are in great danger. So he tries to change the course of events. He might start to make different decisions in his present to avoid the bad future he has witnessed. For example, he could try to resolve conflicts with certain characters earlier to prevent future wars.
It could be a new character that Naruto discovers through reading the future. Maybe someone who has a grudge against the Hidden Leaf and is planning a huge attack in the future.
It could be a future version of himself that has become corrupted by power. He sees this future self causing chaos and destruction, and has to find a way to stop his own dark path. Maybe it's a result of his technology getting out of control and warping his mind.
Naruto is the central character, no doubt. Hinata could be important as well. If Naruto sees a future related to his family or his relationship with Hinata, she will play a significant role. Additionally, the Hokage, like Tsunade or whoever is in charge at that time, would be important as Naruto may need to report the future he saw to the village leadership to take collective action.
Maybe Naruto sees himself as Hokage, with the village prosperous and peaceful. He could see his friends by his side, all having achieved their dreams too.
There's also the possibility that the main antagonist is an outside force from another dimension or a powerful entity that was awakened in the future. This entity might see the ninja world as a threat or simply want to conquer it for its own gain. Naruto would need to figure out how to defeat this unknown and powerful foe during his time in the future.
Naruto might meet his future children. He would be able to see what kind of people they are and what their dreams and aspirations are. It would be an emotional moment for him, seeing his family in the future.