Another option is a new villain created just for this fanfiction. A dark wizard who has the ability to control both ninja and vampire powers. He wants to take over both worlds and Naruto and Moka are the only ones who can stand in his way.
Maybe a powerful vampire from Moka's world who sees Naruto as an intruder and wants to get rid of him. This vampire might also be jealous of the bond between Moka and Naruto and tries to break them apart by creating chaos and fighting against them.
In a few fanfictions, it could be a jealous ninja within the village. Maybe someone who has feelings for Naruto and sees Sakura as an obstacle. This ninja could create a false narrative that leads to Sakura's banishment, thus becoming the main antagonist in the story.
In some fanfictions, a new character could be created as the main antagonist. This character might be jealous of Naruto and female Sasuke's growing relationship and power. They try to create rifts between them and stop them from achieving their goals.
Maybe a new organization of rogue ninjas that see Naruto's Rinnegan as a threat to their own goals of taking over the ninja world. They target Naruko as a way to get to Naruto. These ninjas could have some unique and powerful jutsus of their own, making them formidable antagonists.
Obviously, Naruto and Ura Moka are the main characters. Naruto is from the Hidden Leaf Village, known for his determination and powerful ninja skills. Ura Moka, on the other hand, has her own set of abilities and characteristics from her own world.
Often, Orochimaru can be a main antagonist. Since he has his own devious plans in the Naruto universe, in a time - travel scenario, he might oppose Naruto and Tsunade's attempts to change things for the better. He could be trying to use the time - travel chaos to his own advantage, like stealing forbidden jutsu from different eras.
There could be other ninja from rival villages who act as antagonists. They might see Naruto as a threat and target Hinata to get to him during the Chunin Exams. These antagonists would create conflict and danger for Naruto and Hinata, forcing them to grow and rely on each other more.
One possible antagonist could be a powerful wizard or sorcerer who is jealous of Naruto's growing influence and the harem he has. This antagonist might try to break up the group and steal their powers.
In some versions, Orochimaru could be a main antagonist. Since he has his own devious plans and is always looking for power, Naruto and Ino might have to deal with him all over again as they travel back in time. He could be trying to interfere with their attempts to change the past.
Often, Orochimaru can still be a main antagonist. Even in a time - travel scenario, his thirst for power and immortality doesn't change. He might oppose Naruto and Tsunade's attempts to change the past as it could disrupt his own plans for power.
There could be a powerful and jealous ninja from another village. This ninja wants the power of the Shukaku for himself. He sees the relationship between Naruto and the female Shukaku as an obstacle to his goal, so he devises plans to separate them and capture the female Shukaku, making him a major antagonist in the fanfiction.