Danzo could be the main antagonist. If Obito goes back to prevent the Uchiha massacre, Danzo would be a major obstacle. He has his own hidden motives and is willing to sacrifice the Uchiha for what he believes is the greater good of Konoha, which would put him in direct conflict with Obito.
It could be Madara. Even if Obito goes back in time, Madara's long - term plans for the ninja world are still in motion. He might be the main force that Obito has to oppose as Madara's ideas often lead to destruction and chaos.
Another possibility is Kaguya. If Obito's time - traveling somehow disrupts the flow of time related to Kaguya's seal, she could awaken earlier. In this case, she would be a formidable antagonist as she has immense power and a rather different view of the world compared to Obito, who wants to save his clan and bring peace.
One possible plot could be that Obito goes back in time and tries to prevent the Uchiha massacre. He might use his knowledge of future events to stop the conspiracy against his clan. For example, he could expose the real motives of those who planned the massacre, like Danzo, and rally the Uchiha to fight back in a more united and informed way.
A new antagonist could be a time - traveling ninja from another dimension who doesn't want Obito to change the past. This ninja might have his own plans for the Naruto world and sees Obito's interference as a threat. Or perhaps it could be an old enemy of the Uchiha clan who was thought to be dead but resurfaces in this new timeline because of Obito's time - traveling. They could be more powerful than before and have a vendetta against the Uchiha and anyone who tries to change their fate.
One great Obito Uchiha time travel fanfiction is 'A New Chance for Obito'. In this story, Obito travels back in time and tries to prevent the tragedies that befell the Uchiha clan. It's really engaging as it shows his internal struggle to change things while not disrupting the entire timeline too much.
A new character could be introduced as the antagonist. Maybe someone who has also time - traveled and has their own agenda for the past. This character could be trying to preserve the original timeline for some reason, or they could be trying to create an even worse future. They would oppose Obito and Kakashi at every turn, creating a lot of conflict and drama in the fanfiction.
In Obito Uchiha time travel fanfiction, we can explore different 'what - if' scenarios. For example, what if Obito had known the truth earlier? Fanfic writers can take this and create stories full of drama, action, and character growth. It gives a chance to expand on Obito's character, showing his growth from a broken, vengeful man to someone who can make a positive impact on the ninja world.
Frieza would be a great antagonist. If Goku goes back in time to a point before Frieza was defeated, Frieza could be an even more menacing force as Goku might not have all the power - ups he had in the original timeline. He'd have to rely on his wits and basic training more.
Cell could be a great antagonist. Even if Gohan goes back in time, Cell might still find a way to exist. Maybe he adapts his plan to absorb the Androids earlier in time. Gohan would then have to face a more powerful and unpredictable Cell, who could potentially change the timeline to his own advantage.
It could be Zeref. If Natsu goes back in time, Zeref might have different plans or be in a more powerful position at that time. He could be the main obstacle for Natsu to overcome, especially if Natsu is trying to change things related to the dark magic that Zeref wields.
Perhaps an ancient martial arts master who has gone astray. He might envy Kenichi's pure heart for martial arts and tries to corrupt him. This antagonist could use his deep knowledge of ancient martial arts against Kenichi, creating intense battles between them. Also, he might try to use Kenichi to gain more power in his own time period.
It could be a powerful sorcerer from the time she travels back to. This sorcerer might sense that Elena doesn't belong in that time and wants to stop her from interfering with his plans. Maybe he has some dark magic he wants to use to take over the kingdom or town she's in.