There's a possibility that it could be an old enemy who has somehow returned. Orochimaru was always a tricky character. Even if he seemed to have changed, there could be remnants of his old self that resurface in a more powerful and menacing way. Or perhaps someone from the Akatsuki organization who was thought to be dead but actually survived and has been plotting revenge all this time.
It could be a new character. Maybe someone who has been influenced by the dark side of the ninja arts and wants to overthrow the current order. This new antagonist might be jealous of Naruto's success and the peace in the village, and they strive to create chaos.
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.
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.
One thing we might find in this fanfic is the evolution of relationships. Naruto and Sasuke's friendship could have deepened even more, and they might be mentoring the new generation together. Hinata could be playing an important role beside Naruto as his wife, and their family life could be a part of the story. There could also be new characters introduced who are related to the original characters in some way, like a long - lost relative of Kakashi.
You might expect some exciting developments like new powers for the characters or unexpected alliances.
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.
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.
Maybe it's a new character. Someone who is jealous of Naruto's legacy and wants to erase it. This character could be from a hidden clan that has been in the shadows all this time. They see Naruto as a threat to their own plans for domination, so they start causing chaos in the future Naruto witnesses.
It could be a rogue shinobi who was once a student of one of the great ninja masters but turned to the dark side due to some past injustice. This shinobi could have developed a new and powerful jutsu that makes him a formidable foe for the new generation of heroes.
It could be a time - traveling enemy from the future who wants to stop Naruto from changing things. This antagonist might have his own motives for wanting the future to remain as it was, like having power over a ruined Konoha.
It could be a future version of Orochimaru who has found a way to come back even more powerful. Maybe he has developed some new form of immortality and is causing chaos in the future world that Naruto travels to.