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.
Perhaps an old foe that Naruto thought was defeated in the past but has somehow come back in a different form in the future. And this foe is trying to manipulate events in the past through some dark magic or jutsu to ensure his victory in the future. For example, Orochimaru could have found a new way to cheat death and has a grand plan that involves changing the past to make Konoha weaker in the future. So, he becomes the main antagonist as Naruto from the past tries to prevent his plans.
It could be an unknown shinobi from a far - off land who has some sort of vendetta against Konoha. They might see this shinobi causing chaos in the future and start preparing to face him in the present.
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 new villain from the future that somehow follows future Gohan back in time. This villain might be an evolved form of an existing enemy, stronger and more cunning. They could be after the secrets that future Gohan holds about the future technology or the power levels of the Z - fighters in the future.
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.
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.
One possible plot could be that Naruto from the past somehow gets a glimpse of the future of Konoha. He sees new threats emerging and starts training in ways that can counter those threats in the future. He might also try to influence the present actions of his friends and allies based on what he saw in the future.
It could be a corrupted Gem from the past that was somehow locked away and now resurfaces in the future, angry at the changes that have occurred. This Gem might not understand the new world order and wants to return things to how they were.
Perhaps a powerful figure in the Naruto world who senses Naruto's newfound memories and abilities from his past life and sees him as a threat. This could be a rogue ninja or even a hidden power within the ninja villages. They might try to stop Naruto from fully exploring and using his past - life powers.
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.